Originally Posted by Sjondorn

Ah, yes that's same as for me, sorry let me explain better: when you make the booking at Delta's website and pay for it with ecredits, your skymiles is forced in. Then you go to manage your booking and:
1) Remove skymiles account under passenger details, I never had issue here on the website.
2) Verify that your name is correct (i.e. no double middle name). If you need name correction, then contact delta support (over messaging is fine for me). Don't enter FB # yet, leave FFP blank (if you put FB# support will have issues changing the name because it needs to match FB#)
3) Once they correct your name or if your name was correct, enter FB # under Air France or KLM under passenger details.
4) I then close the page and reopen it again to verify everything is correct (i.e. FB# and benefits).

Voila, done.