Received error message from server… when setting up email forwarding on iOS

I am in the process of moving from 1Password over to BitWarden; I have masked email generation through FastMail set up on my Mac OS system, however I get the following error when attempting to do so in iOS:

“Received error message from server: [200 OK] Field ‘forDomain’ in ‘masked email’ cannot be null…

This message appears after I paste the API key from FastMail into the appropriate box. Help is appreciated!