You can add a free SSL certificate for your domain by following these steps:
1) Login to the DirectAdmin control panel.
2) Click "SSL Certificates".
3) Click the "Free & automatic certificate from Let's Encrypt" tab.
4) Choose the hostnames you want an SSL certificate issued for.
5) Press the "Save" button.
Please note that you can only issue an SSL certificate to a hostname that resolves to our IP address. Please let us know if we can help you further.