Question

Why won’t Environment.GetEnvironmentVariable("variableName") get a variable’s value if the call is made from within a webMethod hosted on IIS and it will work if I call it from a console application on the same machine?

Where do I set up those variables to be visible to IIS web services? Should I use the second parameter from Environment.GetEnvironmentVariable(name, target) to get it?

It is actually really simple:

[WebMethod(Description = "Gets the environment variable value.")]
public string GetEnvironmentVariable()
{
    return Environment.GetEnvironmentVariable("VARIABLE_NAME_HERE");
}

And by the way, VARIABLE_NAME_HERE is set at the system and user level.

Answer 1

Read here for more information:

Using System Wide Environment Variables in .NET Application

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Specifically:

What Are System Environment Variables?

Environment variables are strings that save information about the
entire environment in your system. These string values are dynamic and
they can affect the way your system will behave on. Environment
variables can be classified into two main types:

System Variables: They affect the entire system whatever the current
user is. They are defined by Windows and saved in the registry. You
need to be an administrator to be able to modify them. You usually
need to restart your computer to make these changes effective.

User Variables: They affect the current environment of the current
system user. They can be deleted, modified, and added by any system
user. They are used by Windows setup, by some programs, and by users.
Changes to these variables are saved to the registry and be effective
immediately.

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If you try to access an environment variable that does not exist on your machine you will have issues. You must be trying to find a variable that exists on your local machine, but not on your web service’s host machine.