Selecting all text when clicking to a field?

[OLOT]

Hello,

I'm using a form with a subform for some basic data editing. Its for
looking up existing records and then updating various fields on the subform.
The Form Header has an unbound box that I use to type in an account number
and upon exiting that field it finds the record on the form based on
matching the unbound field to the actual account number field on the form.
I then make my entries on the subform and click back up to the unbound field
to enter the next account number.

If I tab to the field, the previous entry is preselected so that typing
anything erases the old entry and starts typing the new entry. But, if I
click on that field, it just enters that field at the location I clicked and
you have to delete the previous entry by either backspace, delete or
dragging the entire field to select it. I like seeing the previous entry in
case I typed the wrong number so I'm not looking to blank the field upon
entry.

How do I / what are my options : for preselecting all the text upon
clicking into that field?

thanks in advance.