Setting environment variable for service
If it is a local Server 2012 environment variable you seek, you can use PowerShell to create a new system or user environment variable make sure you run PowerShell as Administrator.
Machine variable:[Environment]::SetEnvironmentVariable('Name','Value','Machine')
User variable:[Environment]::SetEnvironmentVariable('Name','Value','User')
To check the current environment variables, use the following PowerShell command
Get-Childitem ENV:
Note: you will need to close PowerShell and open a new instance to see the newly created environment variable.