When preparing a template, you will see an Add tab in the top right. Click on it and you will find an option below to add an Attachment or Recipient attachment.
Note: Recipient attachment is an Enterprise plus feature.
Drop a file into the upload box or click on Use External Link and enter the URL you'd like to attach. Title the file or webpage and check to Add to Library if you would like to have easy access to it for future docs.
Decide whether you would like it to be something required for your recipient to view and then click the Add button on the bottom right.
Once you have added all of your fields and supplemental attachments to your template, you can proceed to the Engage tab to add an introductory video that will always introduce your template.