How to add custom attributes within the Attributes LOV of Manage Event Types task?

Summary:

Unable to add new custom attributes of subscription usage event object within Manage Event Types task of Subscription Management to cover entitlement plan

Content (please ensure you mask any confidential information):

In Subscription Management, how do I add new attributes to the LOV in Manage Event Types?

I'm trying to set up an entitlement plan that should get covered by an usage event type. I've created new custom fields to the Subscription Usage Event object via Apps Composer but the newly created attributes are not reflecting as LOV within Attributes subtab of Manage Event Types task.

Attaching a few snapshots for quick reference.