How to execute button_click manually?

My question is: How can i manually execute this function?

Do not execute button click manually. It's purpose to be executed only when button raises click event.

I want to run in once in the constructor?

Extract code which you have in this event handler to separate method and run it both from c constructor and handler:

public Form1()
{
    InitializeComponent();
    DoSomething();
}

private void button_Click(object sender, EventArgs e)
{
    DoSomething();
}

private void DoSomething()
{
    // extract handler code here
}

You can try this:

button_Click(button,new EventArgs());

or you can do

button.PerformClick();

button.PerformClick();

See MSDN

Its better if you transfer the code from button_click to new function, and call that one. Like this:

    void MyFunc()
    {
        //code that was previously in button_click
    }
    private void button_Click(object sender, EventArgs e)
    {
        MyFunc();
    }

    void OtherFunction()
    {
        MyFunc();
    }

That function is an event handler: a function that gets executed as a response to an event, in this case a user event: the user clicked a button.

If you need to execute the very same action without being triggered with an event, the correct approach is move the code that performs the action to a separate method

private void ExecuteMyAction()
{
   // Do stuff
}

private void button_Click(object sender, EventArgs e)
{ 
   ExecuteMyAction()
}

And then you can call safely that method in your constructor

public class MyClass
{ 
   public MyClass()
   {
      ExecuteMyAction();
   }
}

Actually it is a better to keep the code in a handler as lean as possible and move the code that performs that action to another method. In the handler you just need to gather the information needed to perform an action that it is only available when the event is processed and then pass that information to another part of the system that is in charge of processing that information.

For example, if you need the user to input a name in a Textbox when the user clicks a button, the button_click handler should only get the string from the text box and call a method that saves that information, passing the string as a parameter.

That way you reduce coupling by keeping separated the code that performs the action (business code) from the code that commands the action to be performed (which is boilerplate code that depends on the framework you use)

You can call it like this:

button_Click(this, EventArgs.Empty);