Get Environment variable, Create it if it does not exist.

environment.cs

static string GetEnvironment(string environmentVariableName, string defaultValue)
{
    var Parameter = Environment.GetEnvironmentVariable(environmentVariableName, EnvironmentVariableTarget.Machine);
    if (!string.IsNullOrEmpty(Parameter))
        return Parameter;

    Environment.SetEnvironmentVariable(environmentVariableName, defaultValue, EnvironmentVariableTarget.Machine);
    return defaultValue;
}