Create an email template from html_body/text_body. The template is marked api-created and is hidden from the visual template pickers (it cannot be edited in the visual BeeFree editor). When accessing the API through an OAuth connection from a third-party platform acting on another team's workspaces, this endpoint is restricted to trusted developer platforms; if you are using your own API key (acting on your own account), you can write directly.
See Create a template in the Emails Skill, including the expand[]=html_body/expand[]=text_body params that echo the body you just set back in the response.