Sets a custom field value within QuickBooks.
This method requires QuickBooks 2004 or above, with QBXMLVersion set to at least "3.0".
This method will immediately set the value of a QuickBooks custom field, which has been defined by the user from within the QuickBooks UI. Before calling this method, the preceding operation must retrieve the object from QuickBooks, using Get, ObjSearch, etc. Then, you may use SetCustomField to set custom field values (the fields will be modified or added, as appropriate).
Note that invoking this method will create a connection to QuickBooks if one is not already in place.