How do I run a TeamCity build as a specific user

I have a build configuration which deploys my code to a machine. Depending on which machine I am deploying to (e.g. dev/uat/prod), I need to run as a different user.

Rather than hardcoding the username and password in the build files (not really possible as they change regularly for security reasons) I would like to be able to type them in at the point I run the build. I would envisage the "Run Custom Build" in TeamCity would have this option but I can't see anywhere to input that information.

Is there any way to do this (short of remoting into the build agent and changing the user which the build agent runs as)?

Thanks