Yes, you can almost always claim frequent flyer points both after booking and even after the flight has taken place. If you have already booked but not yet flown, you can simply log into the "Manage My Booking" section of the airline's website or app and enter your loyalty number. If you have already completed your journey and forgot to provide your number, most airlines allow a "Retroactive Claim" within 6 to 12 months of the travel date. To do this, you will typically need your ticket number, flight number, and boarding pass as proof of travel. Some airlines even allow new members to claim points for flights taken 30 days prior to joining the program. The process is usually handled through an online form in your loyalty account. It is worth noting that you can only earn points in one program per flight; if you already earned miles in a partner's program, you cannot "double dip" by claiming them again elsewhere later.