Go to this folder:
C:\Users\{username}\AppData\Local\EpicGamesLauncher\Saved\Config\Windows
Open GameUserSettings.ini
Add a line under [Launcher] section:
CreatedProjectPaths=Parent path of one of your projects
Do not include the path of the project itself, only the parent path. It will pick up ALL the projects in that folder as well as all the engines (source or binary) that those projects use.