Note that each users gets a unique activation code, so you need to use Head Matching and then truncate the URL to leave that part off. The final steps would be:
Match Type : Head Match
Goal URL: /index.php?option=com_user&task=activate&activation=
(this is the return URL when people click on the confirmation email)
Step 1: /component/user/register.html
(This is an example. Navigate to YOUR registration page to see what your url is. It might be slightly different based on your menus)