If you have disabled the launcher and want to enable it again, you can do so in the game in the Settings menu. If you have disabled the launcher and are unable to launch the game, you can force the game to use the launcher by adding a command line argument.
On Steam, right click the game in your Library, choose Properties, go to General. Add -launcher in the field for Launch Options.
On the Epic Games Store, click your profile, choose settings and scroll down to the game. Tick the box for Additional Command Line Arguments and add -launcher in the field below.