Affiliate platform does not send HTML emails as not every mail client accepts HTML emails. A better solution is to create a page with the content that you wanted to send to the customer then place the link of this page in the normal email that goes out.
Now, when your customer clicks on this link they come to the page and can see your message. This will guarantee that everyone will be able to view your message regardless of the email client that your affiliates are using.
I also plan to add autoresponder integrations to the affiliate platform soon so you will be able to send your HTML email from the auto-reponder after the affiliate registers.