When opening RDM as "another user" (runas), how to pass through to sessions?

In my company, we have separate administrative accounts to use for admin tasks. We open RDM with Shift+Right Click, then "run as different user", (equivalent to appending "C:\Windows\System32\runas.exe /user:DOMAINS\USER" to the front of the "target" of a shortcut).

I can't seem to find a way to pass these credentials into an RDP session within RDM. I thought that was how "embedded" would work, but that does not appear to be the case. For now, we are using "My personal credentials", along with a "UserName / Password" type. This does work but it seems that there should be a way that doesn't require separate maintenance of username/password. Am I missing it?