Sandbox doesn't have the store so you can't install Package Manager through the store You need to download the WinGet msix, but that has a dependency on VCLibs which is also not present in Sandbox.
If you search for VCLibs you get to this download center page: Download Microsoft Visual C++ UWP Desktop Runtime Package
But the link to install the VCLibs package is broken (I hope to have them fix it) so instead you have to download it from this other page: C++ Runtime framework packages for Desktop Bridge
So here are the steps in Sandbox: In an elevated powershell:
$ProgressPreference='Silent'
Invoke-WebRequest -Uri https://github.com/microsoft/winget-cli/releases/download/v1.3.2691/Microsoft.DesktopAppInstaller_8wekyb3d8bbwe.msixbundle -OutFile .\MicrosoftDesktopAppInstaller_8wekyb3d8bbwe.msixbundle
Invoke-WebRequest -Uri https://aka.ms/Microsoft.VCLibs.x64.14.00.Desktop.appx -OutFile Microsoft.VCLibs.x64.14.00.Desktop.appx
Add-AppxPackage Microsoft.VCLibs.x64.14.00.Desktop.appx
Add-AppxPackage Microsoft.DesktopAppInstaller_8wekyb3d8bbwe.msixbundle