It's pretty simple, you just append your key to the URL as a parameter.  
For example, looking for users 1, 5, and 15:

    http://api.stackexchange.com/2.2/users/1;5;15?site=stackoverflow&key=YOUR_KEY_HERE

<br>
So, it looks like your `HTTP.call` would become:

    HTTP.call (
        "GET", 
        urlString, 
        {params:{site:"stackoverflow", key:"YOUR_KEY_HERE"}}, 
        function (error,result) {
            console.log (result.data);
        }
    );