header CORS "Access-Control-Allow-Origin" missing. Error code: 403

I'm trying to connect the frontend of an app made in Angular to a backend in Java. I'm trying to pass the data through an API and every time I try to do a post I get the following error:

missing header CORS "access-control-origin" status code: 403

Here's the code of the page I'm trying to do the post from:

import { Component, OnInit } from '@angular/core';
import { NgForm } from '@angular/forms';

@Component({
  selector: 'app-login',
  templateUrl: './login.component.html',
  styleUrls: ['./login.component.css']
})
export class LoginComponent implements OnInit {

name = "";
surname = "";
birthday: number = 0;
email = "";
nickname = "";
password = "";
pasDue!: string;
NameAlready!: string;
passAlready!: string;
constructor() { }

public onSubmit(form: NgForm){
    this.name = form.value.FullName;
    this.surname = form.value.Surname;
    this.birthday = form.value.birthday;
    this.email = form.value.eMail;
    this.nickname = form.value.Nickname;
    this.password = form.value.Password;
    this.pasDue = form.value.passwordDue;
    let nickname = this.nickname;
    let birthday = this.birthday;
    let email = this.email;
    let name = this.name;
    let surname = this.surname;
    let password = this.password;
    const url = `http://localhost:8080/api/v1/user?user_name=${this.nickname}&password=${this.password}&name=${this.name}&surname=${this.surname}&birth=${this.birthday}&e_mail=${this.email}`;
     //console.log("url"+url);
     try{
     fetch(url, {
       method: "POST",
       mode: "cors",,
       headers :{
         "Content-Type":"application/json",
         "Access-Control-Allow-Origin": "*",

       },
       body: JSON.stringify({ nickname, birthday, email, name, surname, password }),
       }).then((resp)=>{
       resp.json();
       }).then((data)=>{
       console.log(data);
       })}catch(err){
       console.log(err);
    
       }
  }
  public onSubmitNew(form: NgForm){
     this.passAlready = form.value.passswordAlready;
     this.NameAlready = form.value.eMail;
  }
  ngOnInit(): void {
  }

}

I built the database and also the backend. What am I doing wrong? I heard that it can be a cross-domain issue. If so, how to solve it?

EDIT: After the suggestion of carloliwanag I share with you the code of the controller class in spring boot which handles the calls to the database:

package com.example.demo.User;

import jakarta.transaction.Transactional;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping(path = "api/v1/user")
public class UserController {

    private final UserService userService;

    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }


    @GetMapping
    public List<UserOfPopster> getStudents(){
        return userService.getStudents();
    }

    @PostMapping
    public void registerNewUser(@RequestBody UserOfPopster user){
        userService.addNewUser(user);
    }

    @DeleteMapping(path = "{user_id}")
    public void deleteUser(@PathVariable("user_id") Long id){
        userService.deleteUser(id);
    }

    @PutMapping(path = "{user_id}")
    public void putUser(@PathVariable("user_id") Long userId, @RequestParam(required = false) String name, @RequestParam(required = false) String eMail){
        userService.updateUser(userId, name, eMail);
    }

 }

So should I add the enableCors function here? Can you please give me some examples of how can I structure it? (Please, don't write the function for me, just give me an example since I never wrote any of these functions before).