If you want to ask your customers "How did you hear about us?" when they make a booking, you should create a Dropdown list custom field.
1. Go to Settings>Customer Details
2. Go to the Custom fields tab
3. Click on the New field button
4. In the Type section, from the Type [1] scroll down menu, select the user picks one of the available choices
5. In the Application section, tick the customer making the booking checkbox [2]. Do not tick the participants [3] checkbox.
6. In the Details section, input the Name [4] and Description [5] of the field.
7. Tick the Public [6] checkbox.
8. From the Default [7] scroll down menu, select if you want to do not set a default choice
9. Click on the New choice [8] button to create a new choice for this custom field. A pop up window will open.
10. Input the Name and Description of this choice.
11. Repeat for other choices.
12. Save