How to auto fill-in in Access Form?

My form has two fields:

Codes: MTN, PRO, OTH, SET, and Raw, etc…

MaintDTCode: any number from 1-999.

My problem is I want to associate (link) the Code
automatically when the corresponding MainDTCode is entered in the form field (e.g. 78=RAW).
Is this accomplished via a table or initial setup of form? If not, how?