javascript - JS Statement for Client Side Alert in .aspx.vb -

i new hard coding & vs. have created couple applications using .aspx beginning understand little more.

i have form (.aspx) has yes/no dropdown box, if user selects yes, it's necessary populate following empty text box (textbox13 "reason/comment"). , need add client side alert message (using js, have found answer) have tried using samples have found. problem is, none of examples need.

so far, did create .js page:

$('#mp_form').submit(function(e) {   if(!$.trim($(this).find('textbox13'="text"').val()).length){       e.preventdefault();      alert('if critical, must provide reason/comment.');     } }); } 

i added in master page, , understand need statement in aspx.vb page function. think onsubmit want not sure how write if, statement. can please me out?
thanks! kathy

have @ following jsfiddle. there's input , button. text in input use following:
$('#inputid').val();
in case inputid comment, in case it's $('#comment').val();