>My question was about form's control. It should be prefilled, I experimented with KeyPress, but don't like the result. I'm now experimenting with GotFocus...

Depends on what you were trying to do in the KeyPress. For one, you should have something like (pseudocode follows)

do case
   case this.selstart>2
{allow everything}
   case this.selstart=2
{allow everything except backspace, home and left arrow}
   other
   this.value=stuff(this.value,1,2,"A0")
   this.selstart=2
   nodefault
endcase