You have a promocode but you don't know how to activate it.
Let me explain ;)
First, you need to be admin of your team. If not, you can not access to the "Billing" section.
Then, at your left, you will find the "Promocode" section. Click on it, type your promocode and press "Enter".
It is done ;)
Now, you can do your subscription and it will be automatically applied.
Do not hesitate if you need help 😉