If you're connecting to the correct port, you'll get a prompt such as
at which point you enter your TSO user id.
Please be aware that most z/OS systems reserve port 23 for 3270 sessions and you cannot connect a telnet session to port 23 for these systems. The data transferred is different and incompatible. If you are attempting to connect to port 23, you need to contact your site support group and find out which port(s) are used for telnet sessions.
I need to test the telnet connection to the zOS agent (it uses USS). I believe it should be ok to test with usual cmd commands such as ping. But could I run jobs/force TSO commands from that telnet? and is there any way to prevent that?