Hello,
We as IdP are trying to integrate FedCM, our production domain has this pattern https://login.idp.com/ - I understand that well-known file must then be served from eTLD+1 so the URL would be https://idp.com/.well-known/web-identity.

But within the development environment our domains have this pattern https://login.dev.idp.com/ (dev.idp.com being a private DNS, not publicly accessible) and because dev.idp.com is not registered in the eTLD list (Public Suffix List) we are unable to properly test or implement FedCM. I guess the navigator requests a well-known file at https://idp.com/.well-known/web-identity.

We need the browser to send requests to https://login.dev.idp.com/.well-known/web-identity. How do we do that? Is there any special flag or configuration we can use in the dev environment to satisfy the condition:

well known file must be served from /.well-known/web-identity eTLD+1 IdP.

The #fedcm-without-well-known-enforcement flag does not work because the error is The fetch of the well-known file resulted in a network error: ERR_NAME_NOT_RESOLVED and not that the returned URLs providing the configuration files do not match.