Go into Engrave mode.
Select the first element on the page and Cmd/Ctrl-click the last barline on the page and then press Make into Frame.
What the “Make into Frame” button is actually doing behind the scenes is this (you could do this manually if you wished) :
Select the first element on the page (a note, for example) and Make Frame Break (left panel) - Shiftl-F
Select the last barline you want on the page and Make Frame Break
Select the Frame Break you created (purple signpost - if it’s not showing then go View > Signposts > Frame Breaks) and in the bottom panel toggle the switch for “Wait for next frame break”.
edit: Anders’s advice may do exactly what you want, but you may find that if you just add a frame break Dorico will reflow music from a previous page (if there is a previous page). If that happens, you’d do better to follow my instructions.