populating field

[rrmangus]

I want to populate a field based on what the user has entered into a field on the same form. I'm familiar with subforms but in this case I want to filter the data submitted to extract only the 3rd, 4th, 5th, and 6th digits from a 7 digit charge code.

I have created a query that does this using the mid function and the field name is as such:

finClass: Mid([TCHG#],3,4)

But now I don't know how to use that same idea in a subform.

Hopefully someone can give me some pointers before I go completely nuts.

Thanks!