Please not that that supposed Free offer is for New subscribers to the Adobe plan and it is to give you either 3 months free on top of a one year plan, 12 payment for 15 months of use, or for a one year plan with only 9 payments.
It is NOT a Completely Free 3 month plan.
It is based on you Signing up for a one year plan.
As to the code can you Type over what is already filled in?
Word of caution. Anytime you Enter your CC number or some other form of payment method what you are signing up for IS NOT FREE.