While your frequent flyer number will be added by default if signed in your online account and booked directly with the carrier, not all flights are booked that way.
For any cash booking by someone other than you especially, you may want to add your frequent flier number or create a free account & add it to your booking to freely earn points. Doing so could allow you to use this trip to earn miles toward your own future travel. United, Delta, Jetblue, & Southwest miles don’t expire, while American Airlines (AA) miles do without points earning activity, the easiest of which is to set up a bank account with Bask Bank that earns 1 AA mile per dollar annually.
Miles earnings, if you attach your frequent flyer number to the booking for $ bookings, is based on the amount of $ spent on United, Delta, American, Jetblue, & Southwest. Unless you’re an elite member, you’ll earn 5x points per dollar spent on Delta, United, & AA. Blue basic bookings on Jetblue earn 2 miles per dollar while blue & above earn 6 miles/$. For low-cost high mileage bookings, you might consider crediting the miles to a partner carrier instead for higher value potential. For AA & Jetblue bookings, look at this free user friendly option unavailable with other carriers.
Miles redemptions for 1-way flights with the right dates/times start at a few thousand SW miles (i.e. VA-FL), 2700 Jetblue miles (i.e. VA-FL during a fare sale), 4500 Delta miles (i.e. a direct flight to NYC from any of our 3 local airports), 6000 AA miles (6500 can get to CA from VA), & 7000 United miles. Keep in mind that the amounts vary by date and can change over time & that these are the last time I checked on 7/2021.