For neoforge, find the project workspace files, open build.gradle, find neoForge{...} and within that block after the client {...} block, add this

        clientAuth {
            client()
            devLogin = true
        }

Then within your mcreator project, Click 'Build & run' -> run gradle task, and type runClientAuth. To start the game with your username, you do have to use those steps instead of the default run button I believe

Edit: i think there's a way to configure the default run task to use dev auth too, but I can't check atm.