External lookup in CRM (as in Books)

Hello

Context: We have a CRM module, similar to Deals, but for Purchasing. Once the PO is created, a link to this PO should be updated in that "deal".

In Books, you can add a custom 'external' field which can look up into CRM modules.

I'm asked to have the same from CRM: To have a lookup field for Purchase Order from Books (or from Zoho Finance's Purchase Order). Is there such field ?

If not what would be the workaround ?

Thank you !