You need to disable "Real-time protection", like this :

• Run Settings > Privacy & security > Windows Security
• Click "Virus & threat protection"
• In the Virus & threat protection window, under "Virus & threat protection settings" click "Manage settings"
• In the Virus & threat protection settings window, set "Real-time protection" and "Dev Drive protection" to Off.

The effect is immediate, no reboot required. You should undo this when you're not playing the game.

To disable Defender from checking in the background while you're using the computer, you need to disable "Real-time protection", like this :

• Run Settings > Privacy & security > Windows Security
• Click "Virus & threat protection"
• In the Virus & threat protection window, under "Virus & threat protection settings" click "Manage settings"
• In the Virus & threat protection settings window, set "Real-time protection" and "Dev Drive protection" to Off.

The effect is immediate, no reboot required. While this setting is in effect, Defender won't do any checks, although it's still possible to manually start disk scans.

You should undo this when you're not playing the game.