How to get environment variable?

I have an MSBUILD pre-build event that looks at the %COMPUTERNAME% system environment variable and copies different app.config files based on the computer name.

I am passing env.COMPUTERNAME as an enviroment build parameter in my build configuration:

Environment variables

When my pre-build event runs it is unable to get any value from either %env.COMPUTERNAME% or %COMPUTERNAME%

The build log shows it is not substituting the environment variable with any value :

ECHO COMPUTERNAME is %env.COMPUTERNAME%

How do I reference the %COMPUTERNAME% environment variable value for my MSBUILD pre-build event?