Go to sign up.
Or link new calendar if you're already signed in.

Provide the email address associated with your iCloud calendar.

FreeBusy will try to detect your calendar provider.
You can also pick iCloud manually.

Turn on two-factor verification for your Apple ID and create an app-specific password for FreeBusy.
Please record this password somewhere (preferable an encrypted password manager) because you'll need it every time you sign into FreeBusy with your iCloud calendar.

Provide the app-specific password from Step 4.

Click Sign in.