fckeditor : How to add prefix to image URL text field ?

I'm using fckeditor 5.2.1 (I cannot change that).
I don't have /filemanager/ folder.


I need to add a prefix on the URL text field.
When the users choose a image, instead of having /bla/image.jpg
they will have http://www.site.com/bla/image.jpg

where http://www.site.com/ was the prefix added to the URL text field.

How?

Thanks, and I hope it's clear,
MEM