Sign in to your account and manage your profile via this link: https://business.google.com
Click "Bookings" and then "Add a link":
Add your booking link (e.g. yourshop.nearcut.com/book, or your own domain) and click "Save":
Note: You can find your booking link in Settings > My Business > Share. The Share button appears once your shop is fully set up (address, staff and services, opening hours, contact info and a manager). It's greyed out if the shop is hidden from clients:
Click the clipboard icon to copy the link, then paste it into the booking link field as above:
When you're done, the appointments button will show on your Google Business profile:
Note: The update can take around 24 hours to apply, so you might not see the change take effect immediately.