How to pass formcollection using ajax call to an action?

If anyone want to pass additional data to the FormCollection then you can try below.

<script type="text/javascript"> 
function SubmitInfo() 
{
    var id = $("#txtid").val();    
    var formData = $('#yourformname').serializeObject();
    $.extend(formData, { 'User': id }); //Send Additional data

    $.ajax({
        url: 'Controlle/GetUser',
        cache: false,
        type: 'POST',
        dataType: 'json',
        data: decodeURIComponent($.param(formData)),
        success: function (data) {
            $('#resultarea').html(data);
        },
        error: function (jqXHR, textStatus, errorThrown) {
            alert("AJAX error: " + textStatus + ' : ' + errorThrown);
        }
    });
}

$.fn.serializeObject = function () {
    var o = {};
    var a = this.serializeArray();
    $.each(a, function () {
        if (o[this.name] !== undefined) {
            if (!o[this.name].push) {
                o[this.name] = [o[this.name]];
            }
            o[this.name].push(this.value || '');
        } else {
            o[this.name] = this.value || '';
        }
    });
    return o;
};
<script/>

Action Method

public ActionResult GetUser(FormCollection frm)
  {
   int UserId = Convert.ToInt32(frm["user"]);
   // your code
   return Json(data, JsonRequestBehavior.AllowGet);
  }

Refer link for more details.

Try:

$(<your form>).on('submit',function(){
    $.ajax({
        url: "/Register/CompleteRegisteration" + $(this).serialize(), 
        // place the serialized inputs in the ajax call                
        datatype: 'json',
        contentType: "application/json; charset=utf-8",                
        success: function (data) {
            if (data.result == "Error") {
                alert(data.message);
            }
        }
    });
});

Since FormCollection is a number of key-value pairs, JSON is inappropriate data format for its representation. You should use just serialized form string:

var form = $("#onlineform").serialize();
$.ajax({
    type: 'POST',
    url: "/Register/CompleteRegisteration",
    data: form,
    dataType: 'json',
    success: function (data) {
        if (data.result == "Error") {
            alert(data.message);
        }
    }
});

Key changes:

1. type of the request set to POST (not necessary here, but seems more natural)
2. Serialized form instead of JSON string as request data
3. contentType removed - we are not sending JSON anymore