Go to the portal environment of Goose: https://my.goosevpn.com/login
Log in here with the account you are registered with at Goose.
After you have logged in, go to > Profile under the My payments you will see change payment method ⬇️
When you have selected change payment method you will see the following screen ⬇️
Choose a payment method you want to add and click Add.
Then follow the steps to transfer an amount of €1. This amount is a verification amount.
Succes, you now have changed your primairy payment method.