Open a popup with a link which auto-adds something in postbox

[Symen_4ab]

Hi there !
Well, I know the title is a little bit confusing, but I didn't know how to say that..
So, here's the long explanation of what I'd want :

On newreply.php (and newthread.php) I have a link which opens a popup, in this popup, I display something I would like to be inserted in the textarea of the post I'm curently writing.

I'd link to display a link which, when clicked, would add the text directly in the textarea.

Exactly the way the smilie popup works, in short..

So, my question is, what would I need to do to achieve this result ?
I'm lost when it comes to javascript, php is more my domain

Thanks in advance !

Update : First thing I did (after hours of research on this) was getting the vb_editor_id of the textarea (Vb_Editor_001 for exemple).
This is now passed as an argument in my popup.

I tried to examine source codes of many pages (smilies, mainly), but I cannot see how smilies are inserted in the post..
I mean, no onclick action, no link at all...
I'm really lost, I tried by using other javascript codes (not vbulletin's), with no results so far...
I can include all vbulletin's javascript files if I need to, but I would really like to know which javascript function is called (and how) to add the text..
Please ?