It's pretty simple, you just append your key to the URL as a parameter.
For example, looking for users 1, 5, and 155:
http://api.stackexchange.com/2.2/users/1;5;151;5?site=stackoverflow&key=YOUR_KEY_HERE 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); } );