functions.php

<?php

/**
 * Shared Counts - Add username to twitter share
 *
 * @see https://www.billerickson.net/code/shared-counts-twitter-username
 * @author Bill Erickson
 *
 * @param array $link
 * @return array
 */
function be_shared_counts_twitter_username( $link ) {
	if( 'twitter' == $link['type'] ) {
		$link['link'] .= '&via=billerickson';
	}
	return $link;
}
add_filter( 'shared_counts_link', 'be_shared_counts_twitter_username' );