XF 1.2 Message After the First Post

Hello All,

I'm a long time vBulletin user who has recently switched over to XF. In vBulletin I had a fake post that would show up just after the first post where I would put a call to action for guests to register. I'm trying to figure out how to do this in XF.

I've done some searching and it seems like this code will place something after the first post
<xen:if is="{$post.position} % {$xenOptions.messagesPerPage} == 0">></xen:if>
and this will make it show up to just visitors.
<xen:if is="!{$visitor.user_id}">></xen:if>
But I'm not sure if that's correct or if it is what template to put it in.
Also, any pointers on how to make it match my post template?

Thanks for any help you can provide.