After slicing, drag the slider on the right to the layer where you wish to stop. Right-click the slider and add a pause. Manually cancel the print when it pauses.
I suppose there is custom G-code that could be inserted at that layer to abort the print, but I don’t know what it is.
Re-starting after a pause (to change filament, or to insert a nut or magnet in a recess) can be done from Studio or the Handy app.