There is no way you can do that via the dialogue and options screens available in Nabble, so you would need to edit the "send_registration_email" macro. It contains the text of the email that people receive asking them to confirm their email address. That requires both some NAML and some HTML coding skills.
Otherwise the best you can do is either point out where the option to subscribe is in the app description field and encourage them to subscribe or switch on the option at:
Options > Application > Extras & add-ons > Email & Notifications > Notify me when someone registers
then you can manually email them with a link to the Subscription page and a request to subscribe.
EDIT: Or you could edit the Registration page macro itself and add a link to the subscription page - but that would risk them never coming back to register!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.