Tying Google Analytics conversion metric to Google Optimize experiment through segment

Question

We are running an A/B testing experiment through Google Optimize where the PRIMARY OBJECTIVE is a Google Analytics Goal:

What we are measuring is how many users trigger a custom Signup request event, with certain UTM properties.

The conversion event is being fired via Segment from our backend, and the data is being sent to a Google Analytics destination.

We have confirmed that the UTM data is being sent to Google Analytics (GA) correctly following Segment's documentation on how to send UTM parameters to GA. So, for example, if we trigger a segment event whose body looks like this:

{
  "context": {
      "campaign": {
          "name": "search_brand",
          "source": "google",
          "medium": "cpc"
      }
      ...
  },
  "event": "Signup request",
  ...
}

We will see a conversion in our GA dashboard that looks like this:

However, this conversion does not get associated with our Google Optimize experiment:

This is understandable, since we have not sent any data as part of our Segment event indicating that the event should be tied to the experiment (e.g., an Optimize container ID or an experiment ID). What we are missing here is how do we associate our Segment event with our Google Optimize experiment?

There are these docs which state:

To integrate with the Google Universal Analytics Optimize plugin, insert your Optimize Container ID in your destination settings.

We have done this, but this hasn't solved the problem, and appears to just be an alternative way to deploy the Google Optimize scripts/snippets on the client side (I am not sure though, because we have not seen any noticeable effect of including this ID, and the documentation is sparse).

This blog post from Segment also briefly mentions integration with Google Optimize and says:

By using Segment and Google Optimize, you can quickly run your tests using custom dimensions, custom metrics, and content groupings.

But does not go on to explain how to use these tools together or link to any documentation.

Is there any parameter we need to be passing as part of the Segment event body that we are missing here to tie these tools together? Thank you!