Alright the forms do work, but still, I don’t want the extra fluff for something so simple. My pages went from about 7 files being download to 20+. I will just backwards engineer the json being posted to klaviyo if there’s not an easier way to do this. It’s pretty silly if this simple functionality isn’t available to implement. 

Here’s the magic incantation from the server side for any future readers. Just replace the three variables indicated by angle brackets. Still annoying that you can’t just do this through an html form without some client side javascript! It’d be so easy!

curl --request POST \
     --url 'https://a.klaviyo.com/client/subscriptions/?company_id=<COMPANY_ID>' \
     --header 'content-type: application/json' \
     --header 'revision: 2023-07-15' \
     --data '
{
  "data": {
    "type": "subscription",
    "attributes": {
      "profile": {
        "data": {
          "type": "profile",
          "attributes": {
            "email": "<EMAIL>"
          }
        }
      }
    },
    "relationships": {
      "list": {
        "data": {
          "type": "list",
          "id": "<LIST_ID>"
        }
      }
    }
  }
}
'

Hey @msl there are plenty of other form solutions that have pre-built integrations into Klaviyo. For example Typeforms and Justuno Forms, where you can sync the submissions into Klaviyo. You also could build on their API if you’re so included.

Also I just did a little digging and found this previous community thread that may answer your q as well? 

https://klaviyo-en.insided.com/sign-up-forms-38/how-to-link-my-custom-form-back-to-a-klaviyo-list-8518