The ways to add your payment details depends on how your profile was added in fitbox.
Below are the different scenarios.
1. Your profile is added manually by your admin directly on the admin portal and sent you and invitation. After accepting the invite you, you'll have to download the app if you haven't already. You'll then accept the gym waiver and answer the health questions. If you or your admin selected a Paid membership for you, the system will ask you to enter your payment details before you can start using the app.
2. After adding your profile, and a Waived membership/Free trial was selected for you, you can just skip the payment page and enter your payment details at a later part by going to App Menu > My Profile > Payments > Add/Update Payment Details > Submit.