Credential w/ Website manipulation

Hi .. Aside from RDP connections that play ball, I have to use my AD credentials from time to time with other systems.

For instance the login to a storage device.
Now I have mastered the skill of pulling out the correct "id" tag to do the auto-fill of the credentials, but I would like to understand how I combine the domain with the username.

e.g. In my 1Password Credential entry I have the username & password stored and I have created a text field for the domain labelled 'domain' (imaginative I know). I have told RDM that information in the Cred entry and for RDP it works like a charm.

However, for the storage device I only have a Username & Password field and it wants the username to be in the format of <user>@<domain>.

How do I specify that in the WebSite Connection entry ?