On Hyper-V, find your virtual appliance IP address:
multipass list
On VirtualBox, forward the appliance port to the outside world with this command in an Administrator PowerShell:
env:USERPROFILE\Downloads\PSTools\PsExec.exe -s $env:VBOX_MSI_INSTALL_PATH\VBoxManage.exe controlvm "<instance name>" --natpf1 "{{ appliance.alias }},tcp,,{{ appliance.port}},,{{ appliance.port}}"
For more information about this command see the Multipass documentation.