As an admin, you have the option in every autoresponder to choose default format for that autoresponder.

If you choose 'Both', the form shows up both radio buttons for Text and HTML formats for subscriber to choose.

If you select either Text/html > radiobuttons will not be showed in the form and subscriber assigned to your chosen default format.

Note: But you need to enter something in HTML textarea on message creating screens. Like a dot or dash ... Just to avoid error of not adding the message.