How to send a form with 2 values ​from javascript?

I have a form and I want from a javascript function to send the form to php with 2 variables that I have in that javascript function

function transfer(old_id){
    var select = document.getElementById('trans');
    var button = document.getElementById('send_id');
    button.addEventListener('click',function(){
        var selectedOption = select.value;
        alert(selectedOption);
        alert(old_id);
        document.delete_user.submit();
    });
}

I want this line (document.delete_user.submit ();) to send the variables to php: selectedOption and old_id

Answer

If you want the form to send the data, then you need to have a form control with the name you want and the value you want.

If the data exists only in a variable (and isn’t already in a form field) then you need to put it in a form field.

Either set the value property of an existing one (which you can get with, for example, getElementById) or create a new one (you’d generally want to use a hidden input for this) and append it to the form.