What is the ruling on the imam's persistence in supplicating via microphone after every prayer, and is it obligatory for the congregants to say "Ameen" to his supplication?

Supplicating after prayers is legislated, but congregational supplication in the form of the Imam supplicating and the congregants saying "Ameen" is an innovation that was not transmitted from the Prophet (peace be upon him) nor from his companions. Persisting in it is an additional innovation. Therefore, it is not legislated for the congregants to say "Ameen" behind an Imam who supplicates congregationally. Rather, they should advise him by clarifying that his action contravenes the Sunnah and that adherence to the Prophet's guidance is necessary.