Forms :: Adding Total Box To A Form?

I am creating a database to track gifts to employees. The admin person will fill out a form for each gift and create a report with a total $ value for those gifts for the history of the employee.

I would like to create a box on the form that totals all of the $'s for the employee selected, so the admin can see what has been gifted before they start the next entry. They will select the employee from a drop-down list (Physician_ID) and when that selection is made, I would like to populate the Total_to_Date box on the form with a total of $ gifted for that employee from the NMC_Detail_Records table.