JQuery :: Making Call On BeforeSend Event Of Another Call?

I need to make a $.post call on the beforeSend event of another $.ajax call.I read the documentation and I see that if the beforeSend return false the ajax call will be interrupt.So I made this code but doesn't work, because the real value of the beforesend function isn't true or false, is the value of the post.I have to do this because when you click on a link I need to verify before if you are able to do this via db permission of my webapp.I think you don't understand...This is the code:

$.ajax({
async: true,
beforeSend: function(){[code]....

});I see always the alert of success event, so the return of the beforesend is not correctly done.