Are you using the URL instant mark up or the IMG instant mark-up.
It sounds as though you are pasting the url down in front of the "http://" that appears in the pop-up box but if I do that I can edit it while in the box! You should be able to edit anything in that box in the normal way while it is still open.

Try pasting the url into the box by just doing Ctrl+V while the "http://" is still highlighted

If that does not work take a look at what you have copied to the clipboard by using clipboard viewer (or by pasting it into a wordprocessor) there should not be an "http://" at the end. If there is it is something to do with the way you are copying it from PB.

Are you using the auto copy facility, ie just clicking on the words "URL Link" ?

If you are trying to put a picture into a forum post just auto-copy the "IMG Code" then just paste that into your post without using the "Instant Mark-up"