MYBB Custom PHP page add to who's online?

[letsforum]

I have many custom page on my forum but don't know how to show in who's online whenever someone is using it?

Here is a tutorial but I have no idea what should I do and where to include the code?

The page I would like to add to online.php is in my signature called "Pagerank checker"

[cronhound]

If you followed this tutorial, it should work without any changes.