How to set custom fields values for Amendments created during Update Subscription through REST API

[KL1]

We have a custom field on Amendments, intended for designating the Update Subscription transaction it was part of (https://www.zuora.com/developer/api-reference/#operation/PUT_Subscription).

I'm not finding a way to populate this custom field when submitting the Update Subscription request through the REST API.

Is the only course of action to fetch amendments on the subscription and update the amendments one by one? However, I see no identifiers for the amendments created during an Update Subscription request in the response.

How are folks approaching populating amendment custom fields?