Nginx allow traffic from any domain

I'm using nginx for the proxy server. My application has a feature where user can use their own domain instead of my domain. For that, they need to point their CNAME to my domain.

This is my Nginx configuration

server {
  server_name                   scan.mydomain.com anonymous.mydomain.com "";
  access_log                    /etc/nginx/log/local-wc.access.log;
  error_log                     /etc/nginx/log/local-wc.error.log;

  location / {
    root                      /var/www/html/qcg-scanning-frontend/dist/webapp/;
    index                     index.html;
    try_files                 $uri $uri/ /index.html;
    proxy_redirect            off;
    proxy_set_header          Host            $host;
    proxy_set_header          X-Real-IP       $remote_addr;
    proxy_set_header          X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header          X-Forwarded-Protocol $scheme;
  }

  listen 443 ssl; # managed by Certbot
  ssl_certificate /etc/letsencrypt/live/anonymous.mydomain.com-0001/fullchain.pem; # managed by Certbot
  ssl_certificate_key /etc/letsencrypt/live/anonymous.mydomain.com-0001/privkey.pem; # managed by Certbot
  include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
  ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

server {
  if ($host = scan.mydomain.com) {
    return 301 https://$host$request_uri;
  } # managed by Certbot

  if ($host = anonymous.mydomain.com) {
    return 301 https://$host$request_uri;
  } # managed by Certbot

  server_name                   scan.mydomain.com anonymous.mydomain.com "";
  listen 80;
  return 404; # managed by Certbot
}

this configuration is working fine when browsed using my domain scan.mydomain.com and anonymous.mydomain.com but using any pointed domain like new.example.com, it gives 404 page (maybe due to return 404 statement).

For SSL, I'm using lets-encrypt certbot.

How can I configure to

1. Allow traffic from all CNAME pointed domains to my server?
2. Provide SSL certificate to all the domains?