Getting postback url w/o iOS AdWords SDK but Firebase only

[DavidV]

Currently, our iOS app uses both Firebase and AdWords SDKs. 

We use the AdWords SDK for the sole purpose of pinging to AdWords on app launch.

The ping triggers a pre-configured postback URL from AdWords to our server, and that's the only way we found to get a hold of the Ad Group of this conversion and match it using AdId to users in our database.

This helps us to determine which Ad Group is the most efficient.

BUT

On AdWords' support page under the title "Track app conversions with the AdWords SDK or a server-to-server connection", there's a note that says "On 03/31/2018, Google Analytics for Firebase will replace AdWords SDK as the primary way to track conversions using an SDK".

This means we need to stop using AdWords SDK.

When you add a Firebase conversion in AdWords panel, as opposed to selecting "First opens and in-app actions", you can't setup a postback URL.

Attached are images of the "old" way with the option to set up a postback URL.

So my question is how can we obtain a conversion's Ad Group with the user's AdId in another way using Firebase SDK only?

Thanks