>>100326226
I don't see a pager, so I don't know how many desks or pages you have, but if you have it set up with 4 desks and 1 page you could get that sort of functionality with:
Mouse 4 R A GotoDesk 1 0 3
Mouse 5 R A GotoDesk -1 0 3
I'm trying to get it working with pages instead of desks, but no luck so far. This link might help https://www.fvwm.org/Wiki/Config/PagesAndDesks/.