apex class Salesforce Event to Slack Notification

Trying to create an apex class in order to receive a notification in Slack when a New Event with the Subject "Demo" is created. 

public class NewDemoBooked {
    private static final String slackURL = 'https://hooks.slack.com/services...';
    public class Event {
        @InvocableVariable(label='Subject')
        public String Demo;
    }
    @InvocableMethod(label='Post to Slack')
    public static void postToSlack(List<Event> events) {
        Event o = events[0]; // If bulk, only post first to avoid overloading Slack channel
        Map<String,Object> msg = new Map<String,Object>();
        msg.put('text', 'The following event has been created: \nSubject: *' + o.Demo + '*');
        msg.put('mrkdwn', true);
        String body = JSON.serialize(msg);    
        System.enqueueJob(new QueueableSlackCall(slackURL, 'POST', body));
    }
    public class QueueableSlackCall implements System.Queueable, Database.AllowsCallouts {
        private final String url;
        private final String method;
        private final String body;
        public QueueableSlackCall(String url, String method, String body) {
            this.url = url;
            this.method = method;
            this.body = body;
        }
        public void execute(System.QueueableContext ctx) {
            HttpRequest req = new HttpRequest();
            req.setEndpoint(url);
            req.setMethod(method);
            req.setBody(body);
            Http http = new Http();
            HttpResponse res = http.send(req);
        }
    }
}

I've got a code coverage of 10% 

Can anyone help me?