Allow adding one ore more checkbox(es) to the Jetpack Form Block for subscribing to a MailPoet newsletter list.
Jetpack forms are easy to use especially if you have Jetpack anyway and are used to the Wordpress block editor. The form itself is created as blocks that are picked the same way in the post editor as any other blocks.
MailPoet would have to add a subscription block to Jetpacks special blocks for the form block.