Troubleshooting Tip: 'This is an invalid domain' while adding a new FIDO key

This article decribes that a new FIDO key is added to a user, a red warning message 'This is an invalid domain.' can appear. 

This error message appears if the FortiAuthenticator GUI is accessed via the IP address and not the FQDN of the FortiAuthenticator. 

 
In order to add a FIDO key, it is necessary to access the FortiAuthenticator GUI via the Device FQDN. Once done, the Security key setup window should pop-up:

Once 'Yes' is selected, the FortiToken-400 security key would need to be inserted into the workstation to proceed with the copying to the hardware FIDO storage Token.