I have slicers in my workbook that I need multi-select enabled by default.
I tried SendKeys.
ActiveSheet.Shapes.
I don't believe you can programatically change that Multiselect toggle as at the time I write this...that property isn't exposed to the user via VBA. Looks like you've either got to go change that setting on all slicers manually, or teach your users to hold down the CTRL key while they make their selections (which lets them make multiple selections as if that toggle was activated).