Add MCE shortcode button for WP content editor.

functions.php

// Register the shortcode.
function my_custom_shortcode( $atts ) {
	extract( shortcode_atts(
		array( 
		  'arg1' => '',
		  'arg2' => ''
	  ), $atts )
	);
	$output = "Whatever you need your shortcode to do...";
	return $output;
}
add_shortcode( 'my_shortcode', 'my_custom_shortcode' );


// Add CTA shortcode button.
// init process for registering our button
add_action('init', 'my_shortcode_button_init');
function my_shortcode_button_init() {
  //Abort early if the user will never see TinyMCE
  if ( ! current_user_can('edit_posts') && ! current_user_can('edit_pages') && get_user_option('rich_editing') == 'true')
       return;

  //Add a callback to regiser our tinymce plugin
  add_filter("mce_external_plugins", "my_tinymce_plugin_register");

  // Add a callback to add our button to the TinyMCE toolbar
  add_filter('mce_buttons', 'my_tinymce_button_add');
}


//This callback registers our plug-in
function my_tinymce_plugin_register($plugin_array) {
  // Find plugin's JS file in site's theme.
  $plugin_array['my_shortcode_button_id'] = get_template_directory_uri() . '/js/my-shortcode-button.js';
  return $plugin_array;
}

//This callback adds our button to the toolbar
function my_tinymce_button_add($buttons) {
  //Add the button ID to the $button array
  $buttons[] = "my_shortcode_button_id";
  return $buttons;
}

my-shortcode-button.js

jQuery(document).ready(function($) {

  // Create MCE plugin
  tinymce.create('tinymce.plugins.my_shortcode_button_plugin', {
    init : function(ed, url) {
      // Register button click command
      ed.addCommand('my_shortcode_button_insert', function() {
        selected = tinyMCE.activeEditor.selection.getContent();
        content = '[my_shortcode arg1="" arg2=""]]';
        if( selected ){
          // Use this to position shortcode with existing selection.
          // Eg. Selected copy can be wrapped with the shortcode open/close tags.
          content += selected;
        }
        tinymce.execCommand('mceInsertContent', false, content);
      });
      var urlEd = url.split('js/');
      // Register buttons - trigger above command when clicked
      ed.addButton('my_shortcode_button_id', {
        title : 'Insert my shortcode', 
        cmd : 'my_shortcode_button_insert', 
        // Use image from theme dir (20x20px).
        image: urlEd[0] + 'images/my-shortcode-button.png' 
      });
    },
  });

  // Register TinyMCE plugin
  // Params = Button ID, Plugin name (match create's first param).
  tinymce.PluginManager.add('my_shortcode_button_id', tinymce.plugins.my_shortcode_button_plugin);
});