Add jmeter user parameter preprocessor via java code

Here is how you can initialise an empty User Parameters element:

UserParameters userParameters = new UserParameters();
userParameters.setName("User Parameters");
userParameters.setProperty(new BooleanProperty("UserParameters.per_iteration", false));
userParameters.setProperty(new CollectionProperty("UserParameters.names", new LinkedList<>()));
userParameters.setProperty(new CollectionProperty("UserParameters.thread_values", new LinkedList<>()));
userParameters.setProperty(TestElement.TEST_CLASS, UserParameters.class.getName());
userParameters.setProperty(UserParameters.GUI_CLASS, UserParametersGui.class.getName());

If you need more assistance with regards to how to fill variable names/values you need to be more specific and explain what configuration you need or attach screenshot of the User Parameters element with your values

Full code just in case:

package com.example;

import org.apache.jmeter.config.Arguments;
import org.apache.jmeter.config.gui.ArgumentsPanel;
import org.apache.jmeter.control.LoopController;
import org.apache.jmeter.control.gui.LoopControlPanel;
import org.apache.jmeter.control.gui.TestPlanGui;
import org.apache.jmeter.engine.StandardJMeterEngine;
import org.apache.jmeter.modifiers.UserParameters;
import org.apache.jmeter.modifiers.gui.UserParametersGui;
import org.apache.jmeter.protocol.http.control.gui.HttpTestSampleGui;
import org.apache.jmeter.protocol.http.sampler.HTTPSamplerProxy;
import org.apache.jmeter.reporters.ResultCollector;
import org.apache.jmeter.reporters.Summariser;
import org.apache.jmeter.save.SaveService;
import org.apache.jmeter.testelement.TestElement;
import org.apache.jmeter.testelement.TestPlan;
import org.apache.jmeter.testelement.property.BooleanProperty;
import org.apache.jmeter.testelement.property.CollectionProperty;
import org.apache.jmeter.threads.ThreadGroup;
import org.apache.jmeter.threads.gui.ThreadGroupGui;
import org.apache.jmeter.util.JMeterUtils;
import org.apache.jorphan.collections.HashTree;

import java.io.File;
import java.io.FileOutputStream;
import java.util.Arrays;
import java.util.LinkedList;


public class HttpRequestWithUserParameters {

    public static void main(String[] argv) throws Exception {

        File jmeterHome = new File("/path/to/your/jmeter/installation");
        String slash = System.getProperty("file.separator");


        File jmeterProperties = new File(jmeterHome.getPath() + slash + "bin" + slash + "jmeter.properties");
        if (jmeterProperties.exists()) {
            //JMeter Engine
            StandardJMeterEngine jmeter = new StandardJMeterEngine();

            //JMeter initialization (properties, log levels, locale, etc)
            JMeterUtils.setJMeterHome(jmeterHome.getPath());
            JMeterUtils.loadJMeterProperties(jmeterProperties.getPath());
            JMeterUtils.initLogging();// you can comment this line out to see extra log messages of i.e. DEBUG level
            JMeterUtils.initLocale();

            // JMeter Test Plan, basically JOrphan HashTree
            HashTree testPlanTree = new HashTree();

            // HTTP Sampler - open example.com
            HTTPSamplerProxy examplecomSampler = new HTTPSamplerProxy();
            examplecomSampler.setDomain("example.com");
            examplecomSampler.setPort(80);
            examplecomSampler.setPath("/api/users/${userId}");
            examplecomSampler.setMethod("GET");
            examplecomSampler.setName("Open example.com");
            examplecomSampler.setProperty(TestElement.TEST_CLASS, HTTPSamplerProxy.class.getName());
            examplecomSampler.setProperty(TestElement.GUI_CLASS, HttpTestSampleGui.class.getName());

            //User Parameters
            UserParameters userParameters = new UserParameters();
            userParameters.setName("User Parameters");
            userParameters.setProperty(new BooleanProperty("UserParameters.per_iteration", false));
            userParameters.setProperty(new CollectionProperty("UserParameters.names", new LinkedList<>()));
            userParameters.setProperty(new CollectionProperty("UserParameters.thread_values", new LinkedList<>()));
            userParameters.setProperty(TestElement.TEST_CLASS, UserParameters.class.getName());
            userParameters.setProperty(UserParameters.GUI_CLASS, UserParametersGui.class.getName());


            // Loop Controller
            LoopController loopController = new LoopController();
            loopController.setLoops(1);
            loopController.setFirst(true);
            loopController.setProperty(TestElement.TEST_CLASS, LoopController.class.getName());
            loopController.setProperty(TestElement.GUI_CLASS, LoopControlPanel.class.getName());
            loopController.initialize();

            // Thread Group
            ThreadGroup threadGroup = new ThreadGroup();
            threadGroup.setName("Example Thread Group");
            threadGroup.setNumThreads(1);
            threadGroup.setRampUp(1);
            threadGroup.setSamplerController(loopController);
            threadGroup.setProperty(TestElement.TEST_CLASS, ThreadGroup.class.getName());
            threadGroup.setProperty(TestElement.GUI_CLASS, ThreadGroupGui.class.getName());

            // Test Plan
            TestPlan testPlan = new TestPlan("Create JMeter Script From Java Code");
            testPlan.setProperty(TestElement.TEST_CLASS, TestPlan.class.getName());
            testPlan.setProperty(TestElement.GUI_CLASS, TestPlanGui.class.getName());
            testPlan.setUserDefinedVariables((Arguments) new ArgumentsPanel().createTestElement());

            // Construct Test Plan from previously initialised elements
            testPlanTree.add(testPlan);
            HashTree threadGroupHashTree = testPlanTree.add(testPlan, threadGroup);
            threadGroupHashTree.add(examplecomSampler, userParameters);

            // save generated test plan to JMeter's .jmx file format
            SaveService.saveTree(testPlanTree, new FileOutputStream(jmeterHome + slash + "example.jmx"));

            //add Summarizer output to get test progress in stdout like:
            // summary =      2 in   1.3s =    1.5/s Avg:   631 Min:   290 Max:   973 Err:     0 (0.00%)
            Summariser summer = null;
            String summariserName = JMeterUtils.getPropDefault("summariser.name", "summary");
            if (summariserName.length() > 0) {
                summer = new Summariser(summariserName);
            }

            // Store execution results into a .jtl file
            String logFile = jmeterHome + slash + "example.jtl";
            ResultCollector logger = new ResultCollector(summer);
            logger.setFilename(logFile);
            testPlanTree.add(testPlanTree.getArray()[0], logger);

            // Run Test Plan
            jmeter.configure(testPlanTree);
            jmeter.run();

            System.out.println("Test completed. See " + jmeterHome + slash + "example.jtl file for results");
            System.out.println("JMeter .jmx script is available at " + jmeterHome + slash + "example.jmx");
            System.exit(0);

        }
    }
}

More information: Five Ways To Launch a JMeter Test without Using the JMeter GUI