To connect your FastMail calendar to FreeBusy:
Provide the email address associated with your FastMail calendar.
FreeBusy will try to detect your calendar provider.
You can also pick FastMail manually.
Type in your FastMail generated App Password. To generate an App Password, you have to go to FastMail Settings -> Password & Security and add an App Password. The generated password will securely sign you in to third-party services like FreeBusy. Please make sure you store the app-specific password in a password manager because most services will display it only one time and you will have to recreate it every time you need to sing into FreeBusy. If you lost the app-specific password, you can always regenerate it from your FastMail account.
Click Sign in.