In your function you have
function Login(_UserName, _Password) {
$(function () {
$.ajax({ // ...
Try changing this to simply
function Login(_UserName, _Password) {
$.ajax({ // ...
You can't step into the ajax function (that's the async bit of it!) but you can do the following to help you debug:
function onSuccess(data) { ... }
...
$.ajax({
...
success: onSuccess,
...
});
and the same for error. You can then set breakpoints within these functions and they will hit.