Option 1

1. In the secondary data source, there should be a orange chain icon indicating the field is linked. If it's not currently visible, click the blue broken chain icon to blend the data.
2. In the secondary data source, add 'Order ID' to Filters.
3. Select Null
4. Select Exclude
5. Press OK

Option 2

1. Create a Calculated Field called "Data source filter" using the following formula (Note: the name of the field from the secondary data source will contain the data source name in the formula):

   IF ATTR([Order ID]) = ATTR([Returns (Sample - Superstore)].[Order ID]) 
   THEN 'Same' ELSE 'Different' 
   END
   

2. Drag both the "Order ID" fields from the primary and the secondary data sources and place them next to each other on Rows
3. Drag the newly created field to Filters and select Same.