This will toggle the Checked label on the selected items:
Properties SETLABEL=Checked SETLABELTOGGLE
To make it like the built-in menus, you might also want to add the Status column if it isn't there already. This will toggle the Checked label and add the Status column next to the Name column when it's missing:
Properties SETLABEL=Checked SETLABELTOGGLE
Set COLUMNSADD=Status(!1+Name)