How to Map a Salesforce Field to a Custom Quickbooks Online Field

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I am using the Opportunity to Invoice functionality in Dbsync, and want to pass the value from a custom field (purchase order) into one of the three custom fields available on the Quickbooks form template, which I have nominated to hold the purchase order number. I have edited workflow rule and mapping to include the custom field in my SQL query, so it is now available on the schema to be mapped, but I am unable to find my custom fields on the Mapping Screen in Dbsync. Any suggestions would be gratefully received. Many thanks