I want to create a custom field on Vendor Bills to track related Purchase orders. This is different from the regular workflow in that the Bill isn't generated from the Purchase Order. The use case is that a standalone Bill is created for a Customs Brokerage vendor and then the custom field(s) would hold the POs of the other vendors that are being brokered across the border - on that bill. What type of custom field would I use and how would I create the link to the purchase order(s)?