error in javascript code... wont add url?

what seems to be up with my code?

<a href="javascript:void(0);" onclick="surroundText('', '', document.myform.yourpost); return false;"><img style="border: none;" src="url.gif" alt="enterurl" /></a>

when click all i get is javascript:void(0);

What is the function surroundText()?

well basicly i want it to do this

like the icons above when B is slected it makes it bold lol thats what this script is.. basicly the bbcode buttons