Create Campaign with Content (not just Recipients)

How do I create and scheduled a campaign using a template and be able to specify content fields like subject, from_name, etc.?

Here’s what I’ve tried:

Step 1: Create the template using POST to https://a.klaviyo.com/api/templates/, noting the template_id from the returned attributes

Step 2: Create the campaign using POST to https://a.klaviyo.com/api/campaigns/, noting the campaign’s message_id in the returned attributes

Step 3: Assign the campaign message to the template using POST (using the two values noted above) to https://a.klaviyo.com/api/campaign-message-assign-template/

Step 4: Update the campaign message with content attributes like subject, from_name, etc. using PATCH to https://a.klaviyo.com/api/campaign-messages/{messageId}/

I’ve tried reversing steps 3 and 4, same result is as follows:

When I look in the UI, the Campaign and Template are created. The campaign “Recipients” info is filled out, but the next step “Content” the Subject Line and Preview Text are empty, and more importantly, the template appears to not be selected.

It looks like in the old v1/v2 API, you could add the content attributes like subject, from_name, etc. directly using a POST to https://a.klaviyo.com/api/v1/campaigns, but it seems unwise to use those when I am warned “v1 / v2 legacy APIs are scheduled to be end-of-lifed on Jan 1, 2024.”