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. For example: Google search, Facebook page, Word of mouth, Airbnb listing, etc.
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.