The way I do it is -

1. Once I get to the payment step on DL's site, I click on "use eCredits" and fill in the info. It will automatically populate the FFP under personal details as "DL SkyMiles," but with the actual # blank. I go ahead and pay for the flight.

2. I go to "manage my reservation." Under "loyalty," the section is blank. I select "Flying Blue" from the drop down box, and type in my number. A few seconds later, the webpage reloads and the system recognizes my # and status "SkyTeam Elite Plus"

3. At no point in this entire process am I logged in. I simply use the website as a guest.