Do you mean an invite code to a federation? There’s only one mainnet federation AFAIK which is the nostr:npub1etgqcj9gc6yaxttuwu9eqgs3ynt2dzaudvwnrssrn2zdt2useaasfj8n6e one. 

fed11qgqrztmhwden5te0v3jkcarp9ekkz6twdejhgtfjx5kk6ctyv45hycfwv3jhvtnxv4jxjcn5vvhxxmmd9upqzgqjppuuzgeh39ne5nkek3a624lcmg752aa5uzelv8a94lf3x7uzfgyzt6t5