Not sure how familiar you are with the PHP code of PHPBB forums
but this is an awful lot of work to make this change to save the user 1/10th of a second to press the left arrow on their keyboard or simply click where you want to type...
It seems to be only a bit of a time saver, one that isn't worth the effort. But from a users view it would be a gain of convenience. Other forum- and also board-systems have this or a comparable feature i.e. often with a JS-prompt for the URL and the optional link text. Many users expect the function and will fail while inserting their messages.
I'm sure, I have seen that feature years ago in a phpBB-driven board before. And yes, like orudge said, this has to be addressed to the phpBB-developers.