How can I pass UTM parameters when subscribing a user to a list via the API?

Sorry for the confusion; I haven’t actually seen how things look in Klaviyo, I’m acting based only on what the client is reporting. The “How they found you” section has to be right.

I see, so it’s not as simple as those fields being filled by the subscription alone; other mechanisms are at play.

Thank you for explaining and for the suggestion. If that’s the only option I’ll go with it, but I’ll wait to hear back from Brian since I’d prefer an option which doesn’t need the Klavio JS to load, out of performance concerns. (In terms of payload alone, I tested loading that script in an otherwise-empty HTML file, and it downloads 48K which decompresses to 106K of JS. That’s a lot just for this one feature.)

Alright, thank you very much.

(Trying again to add a followup here -- I tried once before and it said “awaiting moderation” but then never appeared nor did I get any kind of notification. I didn’t realize marking the answer would irrevocably close the thread, and I’ve had no response from staff either when I asked about reopening it.)

The answer above was not actually a complete solution in this case. The tracking script running on the client did not get the mailing list signup correlated. Looking back it should have been clear to me that wouldn’t work, since the mailing list signup was happening as a back-end process, where of course none of the tracking script’s findings and mechanisms are present.

The complete solution was to not only run the tracking script, but also to make an “identify” call with the user’s email address as/after the signup:

if (window.klaviyo) {
  const emailInput = [...form.elements]
    .find((el) => el.tagName === "INPUT" && el.type === "email");
  if (emailInput != null
      && !emailInput.validity.valueMissing
      && emailInput.validity.valid) {
    window.klaviyo.identify({ $email: emailInput.value });
  }
}