If you are on a rasa.io plan that includes the option to use your own custom HTML template, you can upload the HTML file in the Widgets & Tools section. Custom HTML can be built using attributes as defined on the Email Templates page to provide processing instructions for the insertion of our dynamic, personalized content.
We do not recommend using the custom HTML template option unless you are skilled in HTML coding. Depending on your plan level, customer support for HTML templating is limited. However, if you are not skilled in HTML but interested in having a third party build your template, we are happy to provide recommendations - contact us at firstname.lastname@example.org.