I have a form like this
You can do this, you will need to add ids to items. One thing you might want to do is validate the users input.
HTML
jQuery
$('#myform').submit(function(){
var car = $('#cars').val();
$(this).attr('action', "http://www.mysite.com/" + car + ".html");
});
EDIT: It is best to be javascript at bottom of page;
FULL