Update an email template's subject and body. Cannot update a template linked to an active reminder/promotion mailer. Two update modes: - is_custom=false (recommended): provide 'content' as valid MJML markup. MJML is validated and compiled to HTML server-side; source is stored in content_template. - is_custom=true: provide 'body' as raw rendered HTML. Use this when you want full control and don't need MJML's responsive primitives. Template variables — use ||Variable Name|| in subject or body; they are substituted per-recipient at send time. Event: ||Start Date||, ||Start Time||, ||End Date||, ||End Time||, ||Venue Name||, ||Venue Link||, ||Venue Address||, ||App Name||, ||iOS App Link||, ||Android App Link||, ||Web App Link||, ||Web Site Link|| Recipient (user): ||First Name||, ||Last Name||, ||Full Name||, ||User Name||, ||Email||, ||Phone||, ||Designation||, ||Company||, ||User QR Code|| (renders as QR image URL), ||External Id||, ||Attendee Id||, ||Ticket Id||, ||Ticket Type||, ||Ticket Type Description||, ||Certificate Link|| Also supported: any additional user field defined on the event — reference it by the field's display name, e.g. ||Company Size||.
