[YouTube-API] Where to add Developer key? [Beginner Question]

Hey everybody,

I'm a absolute beginner with the YouTube API and haven't work with many other APIs too. I would like to creat a programm with C# that can use the YouTube API. I would like to add for example a friend. So this is what I got so far:

Service service = new Service("youtube", "YouTube Manager");
service.setUserCredentials("ImGoogleDeveloperatgmail.com", "MyPassword");

string feedUrl = "http://gdata.youtube.com/feeds/api/users/ImGoogleDeveloper/contacts";

FriendsEntry newFriend = new FriendsEntry();
newFriend.UserName = "GoogleDevelopers";
newFriend.Categories.Add(new AtomCategory("Cool Dudes", YouTubeNameTable.FriendsCategorySchema));
service.Insert(new Uri(feedUrl), newFriend);

I get back that "Developer key required for this operation" and the request failed.
Where and how to add my Developer key??

Cheers,
Nick

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