What matters is which airline you are booking the ticket with via Amex, not which metal you are flying. This will show during the booking process where you will see the airline name and icon and if one of flights are being operated by a partner it will tell you that right below the airline name.

For example, say your airline choice is with DL and you are flying ATL-CDG-FRA where the first leg is on Delta metal and the second on Air France. You can buy this ticket from either airline, but you obviously want a Delta ticket. So on Amex search results you need to choose the one that says Delta and below that it says Delta xxxx operated by Air France. This will ensure that you will get the rebate.

I've even gotten the rebate when buying tickets from my chosen airline and flying completely with partner airline. Did US to CDG on Air France and onward in Europe on AF as well. But my ticket was with DL so rebate came through no problem.

What you want to avoid is choosing a result that says multiple airlines or where it does not say your airline choice but one of the flights are operated by your airline choice so you might think it will be rebate eligible but won't be because the ticket isn't bought from the airline of choice.