I see no place to add it!
You need to enter it when you sign-up.
When you activate your first plan, remember to
It's the only time and place where you can do this. If you miss the opportunity while activating your plan, it's too late!
"I see no place to add it!"
If you don't see a box to enter a referral code (just before clicking "Place my order") it's because you already activated a mobile or Home Internet plan with that account in the past.
Create a new account with a different email address (the same card is OK).