The canvas editor gives you a convenient interface to type out any automated subject line or mail text that you want. However, you can include this information in the Wizard or Freeform editors, too!
First, type the text that you want to link. Then, highlight that text in the Wizard editor, and click the link button. In the text box where you would normally insert a URL, type mailto:email@example.com?subject=Awesome subject line!
Replace the email address and subject with what you're going to use, and when your recipients click on that link, an email addressed to you with your subject line will open up.
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