Open the Password & Security → App Passwords screen.
Enter your password and click Unlock, then click New App Password.
Select Custom... name and enter "Leave Me Alone".
Select Mail (IMAP/POP/SMTP) access and click Generate Password.
Copy the App Password - don't forget this bit ️⚠️.
You can now connect your Fastmail account to Leave Me Alone using your email address and the App Password you just generated.