Go to the Reservations & Online Booking section of your account. Here, choose Booking Form Options from the left-hand menu and scroll down to Advertising Sources. Click Add Advertising Source to enter as many sources as you'd like.
Once you've done this, click Save and then head over to the Reservations dashboard.
Click edit next to a reservation/create a new reservation and you will now see a Source field on the bottom-right of the form allowing you to select one of the advertising source options you previously created.