After completing this how-to you will have connected a Netsuite custom field to a Readsoft Online field
Create a custom field in Netsuite.
In the Netsuite target system in Readsoft Online, add to the Custom field mapping: custbody_date_inv_received="%DateInvoiceReceived%".
Where DateInvoiceReceived is the type name of the field you want to connect.
This is what was achieved and what was omitted in this how-to.