Q:

How do I add a custom field on a single page

I need to add a field (text editor) on a Wordpress page, but it's just on a page, whatever specific page by id or slug.
Note: it is a custom field in the WordPress dashboard. It should appear when I edit this page specifies. No plugin, only via code because I do not see the need to install a plugin to do that.
How do I do that?
Thank you in advance! Hail for all!

A:

Hi add this code in your functions.php file 
look like bellow image.

/* Define the custom box */
add_action( 'add_meta_boxes', 'myplugin_add_custom_box' );

/* Do something with the data entered */
add_action( 'save_post', 'myplugin_save_postdata' );

/* Adds a box to the main column on the Post and Page edit screens */
function myplugin_add_custom_box() {
  add_meta_box( 'wp_editor_test_1_box', 'WP Editor Test #1 Box', 'wp_editor_meta_box', 'post' );
}

/* Prints the box content */
function wp_editor_meta_box( $post ) {

  // Use nonce for verification
  wp_nonce_field( plugin_basename( __FILE__ ), 'myplugin_noncename' );

  $field_value = get_post_meta( $post->ID, '_wp_editor_test_1', false );

  // Settings that we'll pass to wp_editor
  $args = array (
        'tinymce' => false,
        'quicktags' => true,
  );
  wp_editor( $field_value[0], '_wp_editor_test_1', $args );
}

/* When the post is saved, saves our custom data */
function myplugin_save_postdata( $post_id ) {

  // verify if this is an auto save routine. 
  // If it is our form has not been submitted, so we dont want to do anything
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
      return;

  // verify this came from the our screen and with proper authorization,
  // because save_post can be triggered at other times
  if ( ( isset ( $_POST['myplugin_noncename'] ) ) && ( ! wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename( __FILE__ ) ) ) )
      return;

  // Check permissions
  if ( ( isset ( $_POST['post_type'] ) ) && ( 'page' == $_POST['post_type'] )  ) {
    if ( ! current_user_can( 'edit_page', $post_id ) ) {
      return;
    }    
  }
  else {
    if ( ! current_user_can( 'edit_post', $post_id ) ) {
      return;
    }
  }

  // OK, we're authenticated: we need to find and save the data
  if ( isset ( $_POST['_wp_editor_test_1'] ) ) {
    update_post_meta( $post_id, '_wp_editor_test_1', $_POST['_wp_editor_test_1'] );
  }

}

