Htek UC903

The phone provisioning is set using the Management top menu and then the Autoprovision left menu

The URL to enter is the simplest one, like

https://demo.mirtapbx.com/mirtapbx/autoprovision/TBP96HnRhZxs7JbP/

In case your certificate is not accepted by the phone, it can be worth trying to allow any certificate or use not encrypted http