All you have to do is go to the backdoor link that Microsoft has so kindly left open on their website:
To silently install the Windows Defender without any user interaction, just append this switch: -qr to the Windows Defender setup command in Run of Start Menu or command prompt. The r switch option will run the Defender setup in reduced UI (user interface) mode, eliminating any need to validate Windows.
Download Windows Defender from the above link, and save it to a convenient location (for example below, I used the C:\ drive).
- Go to Start > Run.
- Key in the full path to your Windows Defender setup file including the name of the setup file (e.g. C:\WindowsDefender.msi if the MSI installer is located in C:\ drive).
- Append and add the -qr to the end of the line. The complete command will look like this: C:\WindowsDefender.msi -qr (or if the location of WindowsDefender.msi has spaces in it's name or path then you may wish to include it in quotation marks: "C:\WindowsDefender.msi" -qr.
- Press Enter or Click OK.
- Wait for Windows Defender installation to complete.