Do not do this. This is a myth.
The “Poor Man’s Copyright” can be faked and doesn’t really prove anything, legally. It does absolutely nothing for you and cannot be used in place of registering your work.
Again, it’s not even necessary to register with the copyright office (in the US anyway) until you publish since you automatically have the copyright to something once you create it. Registering just gives you some extra rights, like suing for money if someone steals your work.