You can password protect individual pages, it’s a built in feature. The related page template could redirect where ever, passing a nonce with the request. The page at where ever could then verify the nonce before displaying content. This scheme would require developing a couple custom page templates to contain the necessary PHP code.
TBH, I don’t care for the password or “code” protected page feature, nor any shared password/code scheme. IMO it’s better to have people log in, registering if necessary, and managing what they can see by role, capability, or other user specific property.
If a shared password should become compromised, you’d have to create a new one and distribute it to all qualified users. It’s simply poor security practice to have a shared password and a PITA. But you do you ?? I get that people don’t always like registering, but it’s the best way to protect everyone involved. FWIW, there are single site sign-on plugins where one sign-on works across multiple sites, like the “Sign in with Google” scheme you often see on sites.
BTW, there are support forums in many different languages, some are fairly active. You may instead post here in your native language. We’ll use an auto-translation service to learn what you posted. While not always 100% accurate, the translation is usually understandable.
