Headless Powermail allows you to render Powermail forms in JSON.
Install extension using composer\
composer require friendsoftypo3/headless_powermail
Then, you should include extension typoscript template, and use provided constants.
Development for this extension is happening as part of the TYPO3 PWA initiative, see https://typo3.org/community/teams/typo3-development/initiatives/pwa/