Either use the Mountain Duck Installer.exe, which automatically performs the shell extension installation,
or install it manually from the package folder:
msiexec /a "$((Get-AppxPackage io.mountainduck).InstallLocation)\Setup\Mountain Duck Support.msi"
InstallLocation usually points to C:\Program Files\WindowsApps\io.mountainduck_<Version>_<Architecture>_nx15cxr8682sp for Website distribution, and a diferent publisher hash for Store distribution.