How can i select all filtered elements in combo

I have a form with a combo dropdown. multiselect is enabled and when filtered i would like the select all button to select only the filtered items instead of selecting all the combo items.

How can this be archived?

Also how can i add a secondary bottom which explicitly says: “Select visible only” ???

Unfortunately the described behavior of the “Select All” button is not supported. It is also not available to place the custom elements to the combo popup.