You can try:
1. Bind external button event/service to SetGlobalSessionBooleanValue
2. Bind the parameter value from GetGlobalSessionValue to ClearSelectionItem of the drop-down widget
so that when the user clicks the button, it can pass the clear selection command to the drop-down widget.
the option to clearSelectionItem is only allowed if the dropdown is not multiselection, how can it be done with a multiselection?