You can just add it to your Delta.com Profile and then select it when booking flights on delta.com. Be aware that DL flights in cheaper coach fare classes (U, T, X, and V) only earn FB miles at 25% of mileage flown (DL Basic Economy E fares earn at 20%). If you are primarily booking cheaper DL coach/BE fares on domestic routes, you won't be earning that many miles. For an infrequent flyer, signing up for their credit card is going to be a much quicker path to earning a significant enough number of miles to actually be useful for something -- https://www.flyingblue.us/en/financi...ank-of-america
For comparison, for a Flying Blue member with no status, you'd need to fly DTW-LAX roundtrip 50 times on U/T/X/V fares to earn the 50K miles that is offered with credit card sign up.