Follow these steps to activate maintenance mode on the site and allow only permitted users to access the site:

1. Log in to Dashboard as an administrator.
2. Click WP Maintenance Mode item in the Settings menu to get a screen similar to the following:

3. Select Activated for the Status field.
4. Select Yes for the Bypass for Search Bots field.
5. Select the user roles allowed to view the backend and frontend while in maintenance mode via the Backend Role and Frontend Role settings.
6. Add any posts, pages, or links to exclude from maintenance mode by using the Exclude setting. You should only add the slug without the other parts of the URL.
7. Click the Save Settings button.
8. Click the Design tab to get the settings related to the design of the screen.
9. Add Title tag, Heading, Heading Colors, Message, and Background Colors using the settings on the screen.
10. Click the Save settings button.

Now, maintenance mode will be enabled on the site, as shown in the following screenshot:

As you can see, the details you added on the Design tab will be displayed instead of the default design and content. Only the authorized users will be able to access all the site's features, while other users will see the maintenance mode screen.