Add Custom Tabs with Options on “Edit Site” Multisite Settings page

/*
 * Add submenu page under Sites
 */
add_action( 'network_admin_menu', 'misha_new_page' );
function misha_new_page(){
	add_submenu_page(
		'sites.php',
		'Edit website', // will be displayed in <title>
		'Edit website', // doesn't matter
		'manage_network_options', // capabilities
		'mishapage',
		'misha_handle_admin_page' // the name of the function which displays the page
	);
}

/*
 * Some CSS tricks to hide the link to our custom submenu page
 */
add_action('admin_head','misha_trick');
function misha_trick(){
	
	echo '<style>
	#menu-site .wp-submenu li:last-child{
		display:none;
	}
	</style>';
	
}

/*
 * Display the page and settings fields
 */
function misha_handle_admin_page(){

	// do not worry about that, we will check it too
	$id = $_REQUEST['id'];

	// you can use $details = get_site( $id ) to add website specific detailes to the title
	$title = 'Misha\'s settings';

	
	echo '<div class="wrap"><h1 id="edit-site">' . $title . '</h1>
	<p class="edit-site-actions"><a href="' . esc_url( get_home_url( $id, '/' ) ) . '">Visit</a> | <a href="' . esc_url( get_admin_url( $id ) ) . '">Dashboard</a></p>';

		// navigation tabs
		network_edit_site_nav( array(
			'blog_id'  => $id,
			'selected' => 'site-misha' // current tab
		) );

		// more CSS tricks :)
		echo '
		<style>
		#menu-site .wp-submenu li.wp-first-item{
			font-weight:600;
		}
		#menu-site .wp-submenu li.wp-first-item a{
			color:#fff;
		}
		</style>
		<form method="post" action="edit.php?action=mishaupdate">';
			wp_nonce_field( 'misha-check' . $id );
			echo '<input type="hidden" name="id" value="' . $id . '" />
			<table class="form-table">
				<tr>
					<th scope="row"><label for="some_field">Some option</label></th>
					<td><input name="some_field" class="regular-text" type="text" id="some_field" value="' . esc_attr( get_blog_option( $id, 'some_field') ) . '" /></td>
				</tr>
			</table>';
			submit_button();
		echo '</form></div>';

}

/*
 * Save settings
 */
add_action('network_admin_edit_mishaupdate',  'misha_save_options');
function misha_save_options() {

	$blog_id = $_POST['id'];

	check_admin_referer('misha-check'.$blog_id); // security check

	update_blog_option( $blog_id, 'some_field', $_POST['some_field'] );

	wp_redirect( add_query_arg( array(
		'page' => 'mishapage',
		'id' => $blog_id,
		'updated' => 'true'), network_admin_url('sites.php')
	));
	// redirect to /wp-admin/sites.php?page=mishapage&blog_id=ID&updated=true

	exit;

}


add_action( 'network_admin_notices', 'misha_notice' );
function misha_notice() {

	if( isset( $_GET['updated'] ) && isset( $_GET['page'] ) && $_GET['page'] == 'mishapage' ) {

		echo '<div id="message" class="updated notice is-dismissible">
			<p>Congratulations!</p><button type="button" class="notice-dismiss"><span class="screen-reader-text">Dismiss this notice.</span></button>
		</div>';

	}

}