Create a custom shortcode in wordpress using functions for any code

functions.php

// reviews widget provided by customer

function shortcodenamehere_function() { 
    $text = <<<TEXT
(Insert any code here)
TEXT;
    return $text;
}
add_shortcode('shortcodenamehere', 'shortcodenamehere_function');
// end shortcode creator

// put this on the page or post:
[shortcodenamehere]