Does a promo code show up on a reservation?

The promo code itself would not appear on the reservation, however by using a specific rate plan (only bookable with the promo code) they will be captured in your reports and easily identifiable. Also for front desk staff the rate plan name appears on the Manage Reservations screen and also if you hover over the reservation in Room Schedule (as shown below). 

You'll see the Rate Plan on Manage Reservations under the RESX side, as shown below.