how to integrate SSL cert with HTTPS post monitor

I have configured HTTPS monitor with SSL certificate alone without Key and it worked fine. even without ssl certificate also monitor is working fine.

You'll probably notice that if you remove the certificate it will still work. This setting is only used when the server requires a client certificate, and as such would also require the private key to function.

Can you tell me, with HTTPS post string.. we need to mention Content type and Content lenght or without these parameters also POST monitor will work?

This depends on the web server, but generally speaking an RFC 2616-compliant POST message would require, at a minimum, the Content-Length and Content-Type headers. The length is the string length of the payload and the type indicates its encoding (usually "application/x-www-form-urlencoded").