Add UTM params to subscribers via form

UTM parameters! They're useful, and we keep making them more useful.

Now, you can supply them even in slightly unconventional ways, like in the form body itself:

<form action="https://buttondown.com/api/emails/embed-subscribe/{{ newsletter.username }}" method="post">
  <input type="hidden" name="utm_source" value="website">
  <input type="hidden" name="utm_campaign" value="spring2025">
  <input type="hidden" name="utm_medium" value="email">
  <input type="email" name="email" placeholder="Your email address">
  <button type="submit">Subscribe</button>

Or in the query string to the embedded form:

https://buttondown.com/api/emails/embed-subscribe/{{ newsletter.username }}?utm_source=website&utm_campaign=spring2025&utm_medium=email

Both will be honored, with the former taking precedence.

We hope you find this useful!