How to insert a BACKDOOR on a system if you have the IP address and some port open ?

Hello.

I would like to know please, to use the payload "windows / meterpreter / reversetcp" for example, must necessarily add a backdoor with "msfvenom" in the target system. But how then add the backdoor with "msfvenom" if there is no access to the target computer system?

How to add a Backdoor with "msfvenom" to a target computer if we have only the IP address and some open port on the target system ???

Thank you.