You probably need to install the guest additions for the Spice protocol that GNOME Boxes uses, see the “guest” section of https://www.spice-space.org/download.html (if your guest OS is Linux it’s a matter of installing the “spice-vdagent” package, if Windows you have an installer for “spice-guest-tools”.
There may also be some blog posts and tutorials about all this on the Internet, here’s an example: https://ctrl.blog/entry/how-to-win10-in-gnome-boxes