Do you want to access the virtual machine from another PC in the network, or (via network) from the host running the virtual machine?
Anyway, by default, Boxes uses NAT. This means that the virtual machine lives in another (private) network that is inaccessible from outside, and the host machine act as a router. Just like your home router: your PC is in a private network that is unreachable from the internet, but via NAT, your PC can access the network.
Probably what you need is a bridge network on the host https://discussion.fedoraproject.org/t/gnome-boxes-bridged-networking/77652
With a bridged interface, you can assign an IP belonging to your network (statically or using the DCHP running in your network).