go to http://www.rogersaccesslibrary.com/OtherLibraries.asp
and then click on Tejpal, A.D. He has a sample database called InputMask_Dynamic.mdb with the following description:
This sample db demonstrates dynamic setting of Input Masks at run time - with
partly pre-filled values as desired.
Three cases are covered -
(a) Phone Numbers - with area codes pre-filled
(b) Dates pre-filled with desired month and year part.
(c) Text string prefixed/suffixed with current month-year.
The sample is in Access 2000 file format developed on Access XP installation.