Is there a plugin to insert pre-defined link into editor?

You can make a button to the Tinymce editor that adds a pre defined link like this youtube button:

Put this in your functions.php theme file:

// Hook into WordPress
add_action('init', 'mylink_button');

// Create Our Initialization Function
function mylink_button() {

   if ( ! current_user_can('edit_posts') && ! current_user_can('edit_pages') ) {
     return;
   }

   if ( get_user_option('rich_editing') == 'true' ) {
     add_filter( 'mce_external_plugins', 'add_plugin' );
     add_filter( 'mce_buttons', 'register_button' );
   }

}

// Register Our Button
function register_button( $buttons ) {
    array_push( $buttons, "|", "mylink" );
    return $buttons;
}

// Register Our TinyMCE Plugin
function add_plugin( $plugin_array ) {
    $plugin_array['mylink'] = get_bloginfo( 'template_url' ) . '/link.js';
    return $plugin_array;
}

And then make a link.js file and put it in your theme root with this in it:

// JavaScript Document
(function() {
    tinymce.create('tinymce.plugins.mylink', {
        init : function(ed, url) {
            ed.addButton('mylink', {
                title : 'My Link',
                image : url+'/mylink.png',
                onclick : function() {
                     ed.selection.setContent('http://mylink.com');

                }
            });
        },
        createControl : function(n, cm) {
            return null;
        },
    });
    tinymce.PluginManager.add('mylink', tinymce.plugins.mylink);
})();

Change http://mylink.com to the link you want to add.
Also add a icon called mylink.png in the theme root.