If your userlist has an id of 5 yes that's correct, but you need to include an input that actually searches something. Example as follows would search the Name field of the userlist with an ID of 5.
Code:
<form action="USERLIST_URL_HERE" method="post">
<input type="hidden" name="listid" value="5">
<input type="text" name="name" value="" />
<button type="submit">Search</button>
</form>
Notice the name text input. This is what's causing it to search the name field.
Code:
<input type="text" name="name" value="" />
Sorry, there's nothing more I can suggest. This is a fully working and tested working example. I cannot help you any further with custom coding needs.