[BirdOPrey5]

You would need to edit your postbit or postbit_legacy template, whichever one you use for your forum.

Search in the template for:

{vb:raw post.message}

That is the text of the first post, so either immediately above or below that enter the code:

<vb:if condition="$post['isfirstshown'] AND $thread['forumid'] == X">
Text Goes Here
</vb:if>

Where X is the forumid of the forum you want to show it.