How to add third party app text contact to phone book favorites? I am able to see WhatsApp, Signal and Viber under Call when adding a Favourite contact, but I couldn’t do the same for text messages. Is this even possible?
This is not a user configurable attribute, but rather how the 3rd party app was programmed. Signal, Viber and WhatsApp shows up under Call as they support receiving VoIP call over lock screen.

There is no way for users to configure the said apps to get listed under Message.