How to add custom fields to the Member Details section for Joint/Family membership?

Please follow the links on how to create a custom field and make it available on the form.
https://help.payments2us.com/m/userguide/l/236183-how-to-add-custom-fields-on-a-web-form
Then add the custom field to the field set on the Payment Item Object->Field Set->externalMemberDetails