MuseScore 3: Format→Style...→Measure numbers (or Bar numbers) ...

I do understand that when setting up a format there is a tick box to exclude a pickup measure from being
included in the measure numbers on the score. However, when I am already a good way into editing a score, and I have identified a pickup I want to exclude from the numbering, the Format→Style...→Measure Numbers route does not appear to get me to where I can exclude the pickup bar from the measures-numbering (on the systems). I have not been able to get the option come up, in a dialog box say, for taking the appropriate action. Nothing I have read (or at least assimilated) so far on the forum or in the MuseScore instructions has resolved this issue for me. I am a relative newcomer to MuseScore so I may be missing what may be very obvious to others. Although I have been trying to follow as much of the really excellent advice in Marc's MuseScore Cafe etc as I can (and I am certainly enjoying and benefiting from his really fine tips and insights) this particular little [seemingly huge] thing eludes me. If you could, kind people, please show a very basic step by step to resolve this niggling problem. Once again, thanks to all of you for the marvellous input into this great ongoing project that is MuseScore