we have a setup with a WordPress website (site a) which uses TheNewsletterPlugin.
we have second website (site b) which uses contact forms.
If a user marks a checkbox for subscribing to the newsletter, we would like to send his data to site a and start the DOI process. any idea how to to this or a script example?
you should put a custom contact form on site b that sends subscriptions to site a. This can be done through Newsletter's custom forms. Check our documentation on how to create one and which options are available.