According to Apple, you have to enable 2 factor authentication and generate “app-specific password” to use the app. You cannot otherwise. https://support.apple.com/en-in/HT202304
Here’s how you do it.
1) Go to https://appleid.apple.com
2) Under the 2 factor authentication section. Click on “Generate Password”. Enter Horcrux and it will generate a text. That should be used as the password in Horcrux Email Backup.
If you need help with setting up your account, please feel free to contact us here. :)