You can use onsubmit
attribute and set the action inside a function for example:
<form id="your_form" onsubmit="yourFunction()">
<input type="text" name="keywords">
<input type="submit" value="Search">
</form>
function yourFunction(){
var action_src = "http://localhost/test/" + document.getElementsByName("keywords")[0].value;
var your_form = document.getElementById('your_form');
your_form.action = action_src ;
}