First, you must get a VirusTotal API key:

1. Create a user on https://www.virustotal.com/
2. Login with your account
3. Click your profile in the top-right corner and click "API key"
4. Copy the 64 character API key into appsettings.json

To register/unregister the handler, right-click VirusTotalContextMenu.exe and select "Run as Administrator".

After registration, simply right-click on a file and select "VT Scan" to scan it with Virus Total.