Unable to Add Custom Field in AR Receipts (Header Level) – AR Object Not Visible in App

In Receivables → AR → Receipts, I am trying to add a custom field at the header level of the receipt. However, I'm running into a few issues:

• I cannot find the AR object in App Composer, so I'm unable to create or expose a custom field.
• The page template for the Receipts UI is not editable.
• I checked the network calls but couldn't identify anything that points to the underlying object used on the page.

Is there any documentation or recommended approach for extending AR Receipts at the header level?
Any guidance on how to identify the correct object or how to enable customization for this page would be greatly appreciated.