Use Contact Form 7 for your Stager mailing opt-ins form.
Stager is an all-in-one platform for ticketing, marketing and event planning. One of its features is a mailing tool for your customers. There is an API to register users for mailings but no WordPress integration to use this API. Until now!
To use this plugin your organisation needs a Stager envirionment, see Stager for more information.
You need to know your organisations Stager subdomain, eg. dkzr when your Stager domain is https://dkzr.stager.co/ and you need an Access Token. In your Stager Settings you will find the necessary information to configure your form.
All the configuration is done in your forms Additional Settings.