You'll find it a few hours into the game when you unlock 'Urki The Thinker' on the map as a potential mission. Go visit Urki and you'll discover it's actually Hurk from Far Cry 3 and 4, complete with bandana and a terrible hick trucker version of the game's caveman language.
His first mission involves an attempt to fly and, after you gather the bits he wants, you get the Easter egg, which you can see in the video below.
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