[AccessD] Editing default field content

I think this is a simple question, and I could swear I've done it before, but I am totally blank and can't get it myself...in my defense, I have been home w/my sick daughter all week, and I'm coming in today, to a pile of backed up voice mails and Email.
 
I have a field that will...almost always...be in the format of AB00###
 
When I enter the field, I want to avoid editing the whole field and be able to just pick up at the tail end of the default text of "AB00"
 
I also want to be able to alter this text, in the event of a uncommon, but not totally rare, occurrence of a different structure.
 
I seem to remember putting in some code for the OnGotFocus event, involving the length of the existing text, but I don't remember what I did, and I can't find it...it is probably in a long-since-replaced program, of old days.
 
If it matters, I am using A2K3, but I originally did this...I do know...in A97.
 
Thanks in advance. I haven't asked a question out here is a really long time, and now I am lowered to asking something simple...or should be as simple...as this ;o)
 
 
John W. Clark
Computer Programmer 
Niagara County
Central Data Processing