Go to 'Settings -> Integrations'. Select MailerLite from the list of integrations.
Enter your API-key and save.
As a final step you will have to select which group in MailerLite you want MakePlans to add new subscribers to. Click on a group and you will see "This service is connected." next to it.
That's it. MakePlans will now automatically add a new subscriber in your MailerLite group whenever a new booking is made.