Custom function after new post/topic

What is the custom function that you are looking to call?

In your main forum directory there is a asp file called new_post.asp

Try looking at that first
The trouble may be (and I'm not being flippant) is you mention this "function", but then it depends on what the function is going to do

Also if you look at the new_post.asp file, you will see that it has these includes:

<!--#include file="common.asp" -->
<!--#include file="functions/functions_send_mail.asp" -->
<!--#include file="functions/functions_format_post.asp"    -->
<!--#include file="includes/emoticons_inc.asp" -->

So adding a custom function (especially when you have not created it yet), might be easier said that done. Good luck with your project though