Originally Posted by
ty97
I've never encountered a party or third party site where could not enter your FF number so I do not anticipate any issues there. If you ever did encounter it, you could request missing mileage credit after the fact, but I really don't think you'll encounter this issue
Delta's Group 1 partners tend to award good earn rates (including MQM bonuses, if you care about that, and usually award miles bonuses too) on their Business fares when crediting to DL. Non-Group 1 partners can vary greatly.
I recently booked a flight directly thru Air Europa, and their website would not let me add my Skymiles number. I will have to ask at the airport or contact Delta after my trip next month.