From the UserVoice item on this:
We evaluate and qualify a curated list of providers who can develop custom controls for Azure AD. You can email "Custom authentication factors onboarding <cafo@microsoft.com>" to add your organization in the waiting list.
Currently we have this capability in preview mode and we have a lot of great feedback from customers on improvements to be made here. Hence the preview is currently closed to new partners.
However, we are actively working on a plan to incorporate feedback and also create a scalable process to onboard more partners.