Integrate Fastmail Calendar
To setup Fastmail first click your initials in the top right of the dashboard > Integrations > Find Fastmail and click Connect:
Then in then you will see a login screen for Fastmail. Sign in with your Fastmail account address and password.
Then click Allow integration the integration:
You will be brought back into the app with a success message. When you have integrated you then can connect that calendar to booking pages: