FYI, Okta provides the password self-service capability for you even if your accounts are AD mastered. This could be a feature that isn't enabled/available by default but it is what we currently use.
That said, prior to this feature being available and tested we pointed the URL to our ticketing page.
Forgotten Password URL: https://ourticketing.com/ForgotUrl
It didn't require changing the "Password managed by" in a different application