How to display something in the first post (only one) of a thread?

[fabioski]

How can I display the result of a plugin, for instance:
echo "hello world";
in the first post of a thread?

I underline the issue of the first post of all thread, I don't want that it apperas in the first post of page 2, page 3 and so on...

Currently I am using this code in the postbit_legacy template:
<vb:if condition="$post[postcount] % $vboptions[maxposts] == 1">
{vb:raw $testo1post}
</vb:if>

but it shows the text of $tasto1post variable in all first post of all pages of a thread.
Thanks.