Using an environment variable to run an external program

Question

I am creating a class library. To test it, I installed Visual Studio to run an external program that uses the class library ( Project Properties\Debug\Start Action\Start external program).

However, Visual Studio does not accept environment variables as part of the path. It says:

Visual Studio cannot start debugging because the debug target "..." is missing. Create a project and try again, or set OutputPath and AssemblyName Properties to indicate the correct location for the target assembly.

Is there a parameter or any other way to enable environment variables?