Ensure you’ve purchased the SSL from external party and got the CRT file.
Once you got CRT file, change the name into demo.example.com-crt.pem for the cert and private key demo.example.com-key.pem and put under following folder:
“C:\Program Files\3CX Phone System\Bin\nginx\conf\instance1”
once done, then all you need to do just restart the nginx service
that’s all