Hi OP, assuming you are running the game under Proton compatibility tools (e.g. GE-Proton10-17, make sure they are all up-to-date), you can use the super secret launch parameter/option in the game properties:
Make sure this is after any instances of "%command%", this can cause issues in cases where for example the game crashes and you wish to provide information about the crash etc.

Of course this will not grant you access to any features the developer has blocked, such as the server browser (Multiplayer) or (for whatever reason) the 'Continue' button, but it will prevent that startup Message.

For testing/running it in the terminal you could try:
Though Steam may not always pass the options forward, so I'd recommend using the client.