In WordPress console go to Settings menu ⇨ Password protection of content. In Password field you can set your password by which protected text will be available.
In option HTML code in place of locked content you can configure your message which will be displayed below password entry form to all visitors. You can add ordinary text (Visual tab) and your HTML (Text tab) here — for example, subscription form code for updating or link to subscription page.
Settings of password and custom text