I have three different flights in my itinerary as follows.
Flight 1: Alaska Airlines
Flight 2 and 3 : Emirates

I am getting below message when I try to buy extra baggage through Emirates manage booking page.
You cannot purchase excess baggage allowance for this flight, because it is not operated by Emirates or you are connecting to/from a flight not operated by Emirates.

Please let me know what are the options I have to buy extra baggage.

Thanks