The problem with your code is that the form continues to submit after you click the submit button so it goes to your "action". To stop that use preventDefault
$(document).ready(function(){
$("form").submit(function(e){
e.preventDefault();
location.href = $('#selectbox').val();
});
});
Note it still won't work in that tool as some of those sites will refuse to load in an iframe, so to break out of the iframe you will need
$(document).ready(function(){
$("form").submit(function(e){
e.preventDefault();
window.top.location.href = $('#selectbox').val();
});
});