Moodle distinguishes authentication -- which creates users, and enrolment which provides users access to courses (this is important for understanding options)
In terms of automatic registration with a form, if the form is populating a database and moodle can access that database you can look into the external database authentication method https://docs.moodle.org/402/en/External_database_authentication for that bit.
Recent moodles include options for course payment with various payment gateways ie https://docs.moodle.org/402/en/Enrolment_on_payment
In terms of automatic registration with a form, if the form is populating a database and moodle can access that database you can look into the external database authentication method https://docs.moodle.org/402/en/External_database_authentication for that bit.
Recent moodles include options for course payment with various payment gateways ie https://docs.moodle.org/402/en/Enrolment_on_payment