Add HTML or Shortcode before or after the_content() - WordPress mu-plugin

before-after-content.php

<?php
// This particular example adds Hustle Social Sharing shortcode after content without editing template files on single posts/pages only.
function jmorris_before_after($content) {
	    $beforecontent = '';
	    if ( is_single() ) {
	    	$aftercontent = '[wd_hustle_ss id="jmorris-online"]';
	    } else {
	    	$aftercontent = '';
	    }
	    $fullcontent = $beforecontent . $content . $aftercontent;
	    
	    return $fullcontent;
}
add_filter('the_content', 'jmorris_before_after');