HELP: auto-populating a field based on previous selection?

HELP: auto-populating a field based on previous selection?

http://creator.zoho.com/kwasserman/shared

Form: "New Job"

The first field in this form is a lookup field that pulls a value from the "MASTER_PROJECT" form, (values = MASTER_PROJECT.MASTER_PROJECT_NUMBER). In the "MASTER_PROJECT" form, the user who input the data originally is prompted to select 'FIRM' and a 'PRIMARY CONTACT' (also lookup fields from other forms). Basically this was designed to force the user to first input Firm info, then Contact info, then Master Project Info, before a new job can be entered.

What I would like to do is automatically update the "FIRM" field in the "NEW_JOB" form upon the selection of a Master Project Number.

How do I script this?
kwasserman