XF 1.2 Show content after post x

[mrb1972]

If I wanted to use this code to place an ad after post x which template would I put the code into?

<xen:if is="{$post.position} % {$xenOptions.messagesPerPage} == x AND !{$message.conversation_id}">
This content will show after post x on every page in a thread
</xen:if>

Thanks

 

[Paul B]

ad_message_below