Try the app?
Accounts - add-ons - entertainment - Disney+
Doing this from the app redirects you to the Disney+ signing page with promos applied. Doing something wrong / out of order seems to be giving people a message their promo code is invalid / already activated.
I did this at launch and had profiles created for family members within a couple minutes.
We are sorry to read that you're having trouble getting the Disney+ offer. We would love to help you get this taken care of. Please tell us more. What issues are you having?