You say field which indicates you are trying to set the property in the
table. if so, click in the row "Input Mask" and press F1 to see the options.
You need to decide if entry is required
AA00000
Or If its not required
aa999999
You will see the available options on pressing F1 in the row.
The same goes for Controls on the form, look for Input Mask in the
properties of the control.
Does this answer your question? If not please re-post here with further info.
Regards
Mike B