Hello,
This sandbox is running on a virtual machine in Hyper-V, not a host machine, right? If so, I'm afraid it's not possible.
The Windows Sandbox is a lightweight virtual environment designed to provide an isolated space to run untrusted applications without affecting the host operating system. Windows Sandbox uses internal networking by default, which means it doesn't connect directly to your physical network or Hyper-V's external virtual switches.
To use an external network in the Windows Sandbox, you must manually specify the virtual switch to use in the registry key of the Sandbox. However, with Windows Sandbox running inside a Hyper-V virtual machine, things are more complicated because you're actually running a lightweight virtual environment inside the virtual machine. Typically, network configuration changes for the Windows Sandbox are specific to the host machine, not the virtual machine.
If you are trying to do this inside a Hyper-V virtual machine, the situation is different because the Sandbox inside the virtual machine does not see the Hyper-V virtual switch directly. In this case, you may need additional network configuration at the Hyper-V level. However, this is generally not recommended because it can break the isolation of the Sandbox.