How to use the Sales Order field on the transaction lines for manual invoices, with validation

Summary:

We manually enter transactions from a third party system. We would like to use the sales order field so that we can use it in an account rule. We currently CAN enter values, but there is no validation on the field. Is there a way to add a value set to validate the numbers being input.

Content (please ensure you mask any confidential information):

I have created an Account Rule that uses the source "Transaction Line Sales Order". I have assigned this to the segment rule override and it works as expected when I manually enter a value in the Sales Order field manually.