When you connect your TWO to your profile in the app for the first time, this year is automatically added.
Did you receive two years of free PRO? Then you activate the second year with the coupon code you received in the email with which you purchased the TWO.
Please note that there should be no active PRO subscription on the account you are logged in with!
If there is, you need to cancel it first before linking the TWO to your account.