Assuming "C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe” is the path to powershell

New-ItemProperty -Path "HKLM:\SOFTWARE\OpenSSH" `
                 -Name DefaultShell `
                 -Value "C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe" `
                 -PropertyType String `
                 -Force