Basic: How to Add Checkbox to New Post Interface?

Good day! In plugin I have to add checkbox onto new post interface which is always checked (it’s value doesn’t have to be saved in database) – how to do that?

Tried mumb-jumbo like

function rt_post() {
add_action(‘dbx_page_sidebar’, ‘rt_admin_sidebar’);
}
function rt_admin_sidebar() {
echo ‘Hello! Some content for admin here: <checkbox name=”check-me” value=”1″ checked />’;
}
add_action( ‘admin_init’, “rt_post” );

but it didn’t work. Any ideas?