Before you can add your required fields in a custom extension you first have to proceed with the configuration steps.
In the steps provided your first extension is part of the configuration process and also the activation of the license through paypal.

For each extension you create you just start the wizard in the configuration chapter. When you have an active license the payment step will not be prompted and you can add your new required fields with a new extension.

When entering the prompt you will open the PayPal payment app and you can select the user needed for accessing this field(s)