I was able to book both DL and UA code shares on the website and didn't have any issue getting the 35% rebate. The DL tickets were flights fully operated by AF and UA was with LH. It's very clear on Amex site who the ticketing carrier is.

If you're not comfortable doing it online, then you can certainly call but I just personally have an aversion to calling them (especially since they once messed me up on a fare class when I was booking with UA and booked me into a basic economy ticket that was not upgradeable so I immediately canceled and then redid the whole thing myself online).