Custom fields are supported in QBO Essentials and Plus on the following QuickBooks Online transaction types:
- Sales Receipts
- Refund Receipts (not supported currently within the Transaction Pro Importer App)
- Credit Memos
You first need to turn on custom fields in QuickBooks Online by going to Gear/Company Settings/Sales.
After turning the custom fields on they will appear at the bottom of the Transaction Pro Importer mapping screen (see screen shot below).