Hello, @accounts_SWS.

When tracking the invoices sent by your vendors, you'll have to enter it as a bill in QuickBooks. As you create a bill, the ability to add a custom field is not available. This is for tracking purposes only since you already have the actual invoice with you. 

As a workaround, you can attach a picture of the actual invoice to the bill. The other way is to use the Memo field or the Description column to add some notes in the transaction.

Get back to us here if you have other questions. I'm always here to help.