When creating Subscription Contracts, there is currently no option to specify "External Document No.".

This creates a problem for customers using EAN invoicing, where this field is required for proper invoice processing.

As a result:

- The generated invoices are missing required EAN-related data

- Users must manually update each invoice after creation

Expected behavior:

It should be possible to define "External Document No." on the Subscription Contract, and have it transferred automatically to the posted invoice.

This would ensure compliance with EAN requirements and eliminate unnecessary manual steps.