How to give a civ access to a unit that does not fall in their tech tree in the scenario editor?

I’ve seen this done quite often in the official campaigns: Joan 1,the British have siege onagers with them, Alaric 3 the Saurus has a Paladin at the gates of Ravenna despite Goths not having access to the Paladin, Joan 5 the French have both Cavaliers and Paladins at once. I’ve tried simply putting the unavailable units there but when I test the scenario, those units usually regress back to a form the civ has available in their tech tree