MetaBox for links/blogroll in wordpress

metaBox.php

<?php
/* Define the custom 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( 'edit_link', 'myplugin_save_postdata' );

/* Adds a box to the main column on the Post and Page edit screens */
function myplugin_add_custom_box() {
    $screens = array( 'link' );
    foreach ($screens as $screen) {
        add_meta_box(
            'myplugin_sectionid',
            __( 'My Post Section Title', '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
  $value = get_option('got_new_key');
  echo '<label for="myplugin_new_field">';
       _e("Description for this field", 'myplugin_textdomain' );
  echo '</label> ';
  echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="'.esc_attr($value).'" size="25" />';
}

/* 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;

  

  // OK, we're authenticated: we need to find and save the data

  //sanitize user input
  $mydata = sanitize_text_field( $_POST['myplugin_new_field'] );


    update_option('got_new_key', $mydata);


}

You are saving the custom field value to the options, this will not work for multiple links. The value for 'got_new_key' will be replaced when adding a new link.

update_option('got_new_key', $mydata);

should be replaced with something like

update_option('got_new_key'.$post_id, $mydata);