Have filtered View while adding Users to a Case Team

Hi all,

In a form for the Case entity I have a field which shows the Users of the Team. When trying to add a new User to the Team, I would like the magnifying lens to show only Users with a certain value on a custom field.
In View Selector I set "Show Selected Views" and then I selected a view which filters only the Users with the custom field set as required, but it still shows all the Users.

Any ideas on if/how I should do this? I'd do it by code too, but I don't have any idea how to access these controls.