Adv Search Select default value

I have created a new custom entity using the advanced customisation wizard for CRM 2018. I checked the box for 'Has Cases' when creating the new entity. The wizard added the new entity ID field to the Cases table as an Adv Search Select with default set to "Search With Current Custom Entity". However, when I create a new case in the context of the new entity, the ASS field remains blank, ie no value defaults.

How do I get the Adv Search Select to default to the custom entity record in context?

Thanks

Paul