Adds Custom Meta Box With Custom Field Using Genesis Hook and Conditional Tag

/** Add custom meta box **/
add_action( 'add_meta_boxes', 'myplugin_add_custom_box' );

// backwards compatible (before WP 3.0)
// add_action( 'admin_init', 'myplugin_add_custom_box', 1 );

/* 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() {
    $screens = array( 'post', 'page' );
    foreach ($screens as $screen) {
        add_meta_box(
            'myplugin_sectionid',
            __( 'Custom Sub-Title Text', 'myplugin_textdomain' ),
            'myplugin_inner_custom_box',
            $screen
        );
    }
}

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

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

  // The actual fields for data entry
  // Use get_post_meta to retrieve an existing value from the database and use the value for the form
  $value = get_post_meta( $post->ID, 'subtitle_text', true );
  echo '<label for="myplugin_new_field">';
       _e("Please enter the custom text that follows the page title.", 'myplugin_textdomain' );
  echo '</label> ';
  echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="'.esc_attr($value).'" size="80" />';
}

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

  // First we need to check if the current user is authorised to do this action. 
  if ( 'page' == $_POST['post_type'] ) {
    if ( ! current_user_can( 'edit_page', $post_id ) )
        return;
  } else {
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;
  }

  // Secondly we need to check if the user intended to change this value.
  if ( ! isset( $_POST['myplugin_noncename'] ) || ! wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename( __FILE__ ) ) )
      return;

  // Thirdly we can save the value to the database

  //if saving in a custom table, get post_ID
  $post_ID = $_POST['post_ID'];
  //sanitize user input
  $mydata = sanitize_text_field( $_POST['myplugin_new_field'] );

   // Do something with $mydata 
   update_post_meta($post_ID, 'subtitle_text', $mydata);
}

/**
* @author Brad Dalton - WP Sites
*
* @link http://wp.me/p1lTu0-9WF
*/
add_action( 'genesis_after_post_title', 'custom_field_before_content' );
function custom_field_before_content() {
if(is_page() || is_single() ) {
  genesis_custom_field('subtitle_text');
  
 }

}