LearningSFDevAPI.cls

public static void sendMessage(Message msg) {
    HttpRequest req = new HttpRequest();
    req.setMethod('POST');
    //req.setEndpoint('https://api.learnsfdevwithapex.com/messages');
    req.setEndpoint('callout:LearningSFDevAPI/messages'); //This is for the final version at the end of the chapter
    req.setBody(JSON.serialize(msg));
    req.setHeader('X-Auth-Token', 'SEE_BOOK_FOR_VALUE');
    req.setHeader('content-type', 'application/json');

    Http http = new Http();
    HttpResponse res = http.send(req);

    if(res.getStatusCode() != 200) {
        throw new LearningSFDevAPIException('API callout returned with a status of ' + res.getStatus());
    }
}