Why I cannot get action from ajax post request

Why I cannot get action from ajax post request

Here is my code:

    $(document).ready(function(){
    $("#mainbutton").click(function(){
    $("#ajaxform").submit(function(e)
    {
       $.ajax(
       {
        action : "mainbutton",
           url : "userctrl",
           type: "post",
           data : $(this).serializeArray(),
           success:function(data, textStatus, jqXHR) 
           {
            alert("success");
           },
           error: function(jqXHR, textStatus, errorThrown) 
           {
               alert("error"); 
               
           }
       });
    });
    $("#ajaxform").submit(); //Submit the form
    });
    });

I try to set action in this post request but when I try to get it from the servlet it is null. Tried to change the request to get and again null. Also tried to put content-type: application/x-www-form-urlencoded but again no success. How to put action in this request. And it is appropriate to put action in post? If not should i make hidden field instead ?