Setting to use git-bash as default shell when connecting remotely via OpenSSH to Windows Server 2016
This should be all you need:
New-ItemProperty -Path "HKLM:\SOFTWARE\OpenSSH" -Name DefaultShell -Value "C:\Program Files\Git\bin\bash.exe" -PropertyType String -Force
If you need to use the cygwin provided bash, this worked for me:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\OpenSSH]
"DefaultShell"="C:\\cygwin64\\Cygwin.bat"