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Activate it on steam, click cd key, put that cd key in uplay and there you go, no need to exchange, it works with almost all of ubisoft games which are on steam.
I could do it (I wanna activate my season pass, which is on steam, but my game is on Uplay). What region are you located in ?
Assassin's Creed Syndicate [Steam, RU-CIS]
Assassin's Creed Syndicate [Uplay, RU-CIS]
Simple exchange