shortcode-for-user-main-blog.php

<?php
// List all user admin sites
// This is correctly listing the primary blog url
function get_primary_blog_url($id=null) {
    // is there a user to check?
    global $current_user;
    $user_id = (null != $id ) ? $id : $current_user->ID;
  
    if ( isset( $user_id ) ) {
      // Get the blogs of this user
      $user_blogs = get_blogs_of_user( $user_id );
  
      // Get the URL of the blog
      foreach ( $user_blogs AS $user_blog ) {
        if ( $user_blog->userblog_id != BLOG_ID_CURRENT_SITE ) { 
          return $user_blog->siteurl;
        }
      }
      // If user has no blog, return to the homepage.
    } else {
      return home_url();
    }
  }

add_shortcode( 'dgtl-sites-listing', 'get_primary_blog_url' );