at the log in screen, there is a small gear wheel on the bottom right side, choose “Pop on Wayland" (edit: if you want COSMIC DE to work too, see update below)
update:
there are some settings on the virt-manager to make COSMIC DE works.
if you don't need to run COSMIC DE, can ignore the following
Display Spice : Type = Spice server, Listen type = none, uncheck password / opengl
Video : use QXL (instead of default Virtio)
new step 8. at the log in screen, there is a small gear wheel on the bottom right side, choose “COSMIC"
cosmic