Hi Koen,
Have you looked at the link below already?
docs.microsoft.com/.../add-extension-properties
This is also another sample to show how to view extension property in CPOS or MPOS:
C:\RetailSDK\POS\Extensions\SampleExtensions\ViewExtensions\SearchPickingAndReceiving\ CustomOrdersListColumns.ts
Actually there may be different reasons why the extension property it is not appearing in CPOS and it needs some deeper investigation to verify you do every part correctly.
Please create a support incident if you need any further assistance.