To prevent your antivirus from deleting the file again as soon as you launch the game, you must create an exclusion.
If the file isn't in your history, right-click the game in your . Go to Properties > Installed Files .
(often flagged as "HackTool" or "Trojan"), select it and choose Add Folder Exclusions : To prevent the file from being deleted again, go to Virus & threat protection settings Manage settings Exclusions . Add the entire game folder as an exclusion. Verify Game Files
Remember: While onlinefix64.dll is the key to unlocking online features in modified games, it walks a fine line. Keep your actual personal data secure, never run the file as "Administrator" if you don't trust the source, and always, always keep your real Windows Defender running for everything except your game folder.
Return to and click Manage settings . Scroll to Exclusions and click Add or remove exclusions . Click Add an exclusion > Folder .