They will give you the code. You must put it into either your portal, or forum description, whichever is on your main page.
You should be using this code:
Code:
<a href="http://www.freedomain.co.nr/" target="_blank">
<img src="http://ezzaoaa.4u.com.ru/but1.gif" width="88" height="31" border="0" alt="Free Domain & Hosting" /></a><br>
You will need to turn HTML ON, and have the following tags as allowed: