With Breadwinner for QuickBooks Online users can map and automatically populate Custom Fields on Invoices, Bills, and Purchase Orders.    

All the procedures to properly configure custom fields in QuickBooks are described in this QuickBooks Help Article by Intuit. 

There are some requirements regarding the custom fields that can be exposed and mapped in Breadwinner. And, in some cases, these custom fields may already exist in QuickBooks:

-     The QuickBooks API limits Breadwinner to the first 3 custom fields (some versions of QuickBooks can have up to 10 custom fields). Please note that these are not necessarily the first three fields in the respective table in the QuickBooks Online Configuration pages, but that this pertains to the first three fields in the order that they were created. So if you have older, inactive fields, then they may have to be "re-activated" and renamed to match the information that they are supposed to contain.

-    The field type has to be "string" (i.e. letters and numbers),

-    The custom fields have to be set as "active" in QuickBooks.

In a recent inquiry with Intuit regarding the custom field creation, Intuit added that in order for the custom fields to show via the API, they also have to be "activated on all sales transactions" and "printed on form"  has to be checked on at least one.