-Go to the "certificates wizard".
-Choose the target web server.
-When you are first prompted for a domain to certify, you need to enter your main domain, e.g. example.com.
-In the next step, you will be able to specify a subdomain for Friendica, if needed.
-Example: If you have friendica.example.com, you first enter example.com, then specify the subdomain friendica later.