Yes, it is possible to use an additional SSL certificate for your Symfony server that refers to your public domain. You can set up a reverse proxy using tools like Nginx or Apache, which allows you to handle SSL termination. Obtain a valid SSL certificate for your public domain (e.g., via Let's Encrypt) and configure the reverse proxy to forward requests to your Symfony server. This way, the third-party service will connect securely using the valid certificate, while your Symfony server continues to run in development mode locally.