According to Authorize.net, the “Subscription Duration” is handled from our end, not from theirs. They do have an option for “No End Date”, but I don’t see this on the DT Donate form as a setting.
We would like the form to send “No End Date” to Authorize.net but the donor not to see that, but rather to see a line that says “To cancel donations, please contact us at XYZ".