var formID = "form_" + "<?=CUtil::JSEscape($arResult['FORM_ID'])?>";
var form = BX(formID);
var sourceEl = BX.findChild(form, { "tag":"select", "attr": { "name": "SOURCE_ID" } }, true, false);
BX.bind(form, 'submit', function(){
var id = sourceEl.value;
if(id == 20){
alert(id);
return false;
}
});