To enable F8 boot in Windows Server 2012 / Windows Server 2016, / Windows Server 2019 you need to go to an elevated CMD prompt.

If your server does not start (hence the need for F8), you can boot from a Windows Server installation DVD and select the option REPAIR YOUR COMPUTER, Troubleshoot, then Command Prompt.

Once you have opened a command prompt (as an administrator), enter the following commands:

Restart your server and press F8