HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Installed HD 323f, but can't find the option to password-protect saves in the settings. Is there a screenshot showing how to do this?

To enable this feature, you need to play with HD+ (option in the launcher). If HD+ is enabled, you won't be able to skip entering the password in online games, and the corresponding option will be available. Whether to play with password-protected saves or not is decided by the host.