You need to use NetworkInformation.GetHostNames.
var hostNames = Windows.Networking.Connectivity.NetworkInformation.GetHostNames();
But note that this method will return multiple HostNames.
In my case, it returns four HostNames of which DisplayNames are MyComputerName, MyComputerName.local plus two IP addresses.
So I guess it's probably safe to do -
using Windows.Networking;
using Windows.Networking.Connectivity;
var hostNames = NetworkInformation.GetHostNames();
var hostName = hostNames.FirstOrDefault(name => name.Type == HostNameType.DomainName)?.DisplayName ?? "???";