I use the Child Relation Editor to link to related tables without issue. In the example below I can successfully link to PRODUCT and use field form PRODUCT on this screen.

I now want to link to product_ext so the link would be stock_serial_id → product → product_ext
How do I do that? I have tried the below.

However I get the error below when adding a field from PRODUCT_EXT so my linking method must be incorrect.

Any help on the correct procedure would be appreciated.
Thanks,
Barry