onChange Textbox Equal to Forms Action

<form action="search.php?q=VALUE" type="post" />
<input type="text" id="textbox" name="poster" size="25" />
<input type="submit" class="button" value="Go" />
</form>

HTML:

I have this form....What I want is for where it says VALUE to change in the forms action to be the same as what is in the textbox.

I cant find anything on the internet that will do this seemlessly without the user noticing.

By the way Im a complete javascript newbie, so treat me gently

 

You don't need to do it, if you're doing a POST. The value of the textbox (poster) will be made availabe in the POST variables.

 

I know but I have some slightly unusual circumstances.

 

<script type="text/javascript">
function UpdateAction(val) {
    document.getElementById("myForm").action = "search.php?q=" + val;
}
</script>

<form action="search.php?q=VALUE" id="myForm" type="post" />
<input type="text" id="textbox" name="poster" onchange="UpdateAction(this.value);" size="25" />
<input type="submit" class="button" value="Go" />
</form>

Code (markup):