Two-factor authentication is mandatory for iCloud Mail, so to add it as a source to Findo you have to do the following:
1. Go to https://appleid.apple.com/#!&page=signin and log in with your Apple ID.
2. Find the security section and if two-factor authentication is off for your devices turn it on.
3. As the two-factor authentication will be on, click generate password button in security section.
4. Enter "Findo" in the window which will pop up, then click OK and an app-specific password will appear.
5. Enter this password into the password field when adding iCloud Mail account to Findo.