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  scroll down menu, select the user picks one of the available choices
5. In the Application section, tick the customer making the booking checkbox . Do not tick the participants  checkbox.
6. In the Details section, input the Name  and Description  of the field.
7. Tick the Public  checkbox.
8. From the Default  scroll down menu, select if you want to do not set a default choice
9. Click on the New choice  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.