Copy One Text Field To Another?

I am looking to take the information submitted into field A and add it to the end of the value of field B

Text Field A = Zip Hidden Field B - redirect = http://mydomain.com?&=<<<ZIP>>>
<form method="post" >
<input type="hidden" name="redirect" value="http://mydomain?l=<<<ZIP>>> "/>
<input type="text" name="Zip" value='' ">
</form>

When they put in their zip code, I need the script to add that information at the end of the redirect URL

Hope that explains what I'm trying to do.