New field will be added into pubkey named "username": https://issues.redhat.com/browse/HMS-1983

The format must be validated, only valid UNIX usernames are possible:

^[a-z_]([a-z0-9_-]{0,31}|[a-z0-9_-]{0,30}\$)$

When user adds a new key, this value can be detected from the third column (comment) of the ssh-key:

ssh-rsa XXXXXX comment

Typically it contains username but not always - the value must be still validated against the regexp. Username must NOT be "root".

Finally, the UI must clearly state this:

Username - account to be used with the SSH key. This field is currently utilized in Google Cloud, other providers do use static username: ec2-user and azure-user.