Gets a custom field value from QuickBooks.
This method will get the value of a custom field, which has been defined by the user from within the QuickBooks UI.
To read the values of custom fields, be sure to set QBXMLVersion to at least "2.0" when retrieving data from QuickBooks. Then, QuickBooks will return custom field information, and it will be accessible through this method.
You may also invoke Config to read the value of custom fields. However, to set the value of custom fields you must set QBXMLVersion to at least "3.0" and invoke SetCustomField.