There is no such thing as "Selections in a multi box". Selections are made on fields, and the Qlik engine does not save the location from which the selection was made.
You can get the full list of selections using GetCurrentSelections(). You can use GetFieldSelections() on each of the fields within your multi box, one at a time, to get the selections for just those fields (of course, those selections could have been made from anywhere, not necessarily from the multi box).
