This feature applies to Mailtrain v1.6+
Mailtrain has a simple API support that allows to add new subscribers to a list, update existing subscribers and mark subscribers as unsubscribed.
You can find the link to API information from the user menu
By default API access is disabled. To enable it you need to generate an api access token using the "Generate Access Token" button
Once you have enabled API access you can see your personal API access token
To make any API calls you also need the list ID. You can find it from the Lists page, next to list name