Connect ExpertSender

1. Sign up to leadthing.dev and purchase a subscription
2. Get the leadthing.dev API key
3. Create an ExpertSender API key (Settings > API)
4. Get the ExpertSender Instance Domain (Settings > API)
5. Get the contact list id where the leads should be added to
6. Send the Newsletter System name, the Expert Sender API key, the Domain, the contact list id and the leadthing API key to Sovendus