Copyright starts automatically as soon as your idea comes into existence.
That said, there is the burden of proof. You need to be able to prove that you were the first with the idea. Sending it to yourself as an e-Mail is a simple option, because it will be timestamped. Just have to hope that the e-mail provider doesn’t shut down until you need the proof. Sending a sealed letter to yourself and keeping it sealed should also work, because the post will put a stamp with a date on it.
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