Subject and sender follow the standard Company Form Request / Company Form Form, as in Telesupply SMS Opt-In Request. Generic wording lands in clients' spam folders.
Auto responder URL must be FQDN and must point at an HTML file, e.g. https://www.example.com/responder-sms-opt-in.html. Its contents are emailed to the submitter, whose address is read from the field named email or Email.
These recipients point at Atomic8Ball. Testing is not finished — switch them back to the client.