BuddyPress.org

To create a shortcode, you use the same function as in the mentionned topic and instead echoing, you return what you need. Like this:

function bpfr_message_shortcode() {	
$author = get_the_author();
$message = 'Login to send a message to this author !';

if( is_user_logged_in() ) {
	return '<a href=" '.wp_nonce_url( bp_loggedin_user_domain() . bp_get_messages_slug() . '/compose/?r=' . $author ) .' title="Private Message to Author" ">Contact the author</a>';
 } else {
    return $message;
 }
} 
add_shortcode( 'ksam', 'bpfr_message_shortcode' );

Add this to bp-custom.php or your child-theme functions.php.

Use the shortcode [ksam] at the place you want and you’ll see a Contact the author link.

Note that I tested it only within a post, meaning you need to be in a post loop to fetch an author.