Because you asked “How to Code”, here is my guide to get you started.
- Create a menu page in the administration area using add_menu_page
- Create the form using html select form-element
- Populate the form-element with all pages. To get all pages you can use get_posts()
- Save the form using admin_post action
- Save the selected-item of the form using update_option()
- To get the saved option you can use get_option()
- To add that — Special Text in the page list, you need to filter the title. If the page’s ID is equal to the saved_setting, then append that text. I’m not sure which hook to use, I think it is manage_pages_columns
I hope it helps.