Autofilling a textbox and entering data in a textbox.

I am trying to have a text box (unit) autofill after I enter data in another
text box (docu nr). Is there any suggestions as to how I can do this?

The docu nr text box contains a a six character code that refers to a unit.
This six character code is the primary key for the unit.
After I type in the docu nr I want the unit to be displayed in the unit text
box.

docu nr format is AAAAAA-1111-1111
unit format is text

docu nr is stored in a table named tblDocuNr

unit is stored in a table named tblUnitChoices

I have been attempting to write a VBA code in the afterupdate portion of the
docu nr text box on my form, but have not perfected it. Any Help would be
appreciated, thanks in advance.

The AAAAAA portion of docu nr refers to a unit.