How to open the launcher if you clicked "Always launch the game without showing this menu”

Open steam, go to DOOM Eternal, and click on the gear icon on the right of the screen. Click "manage", and click "manage game files". Alternatively, right click the game and click "manage game files".

Now you're in the files, open doom_launcher_settings.json and open through notepad just this once. Change the "launch_target" to "portal". Save the file, close, and you're done! Open Doom Eternal and the launcher should be there.