You can enable the waitlist feature in online booking so patients can add themselves to your waitlist. To do so, go to My clinic > Scheduling settings > Online booking:
Scroll down to the waitlist section and check the box next to Enable wait list:
Scroll down further and click Update settings.
When you have enabled the wait list for online booking, your patients will see the following at the bottom of your calendar on your online booking page 'Can't find a time? Join the wait list.'
They will then be prompted to sign in or sign up, and then complete their availability criteria: