This is a duplicate (or similar) question. I already answered the other one under Transportation: https://community.ricksteves.com/travel-forum/transportation/swiss-travel-pass-2ab947b4-56a4-4c65-be34-f2c9603fd88d
"I understand with swiss travel pass (STP) very useful in europe" - It is only valid in Switzerland (and a few cross border lines). NOT the rest of Europe.
"Should i present my STP to the ticketing counter before i hop in the train ?" - No you just get on the train.
Trains do not have "check in" or any other of the horrible features of air travel. You just walk off the street onto the train.
Except the ticketing counter at Zürich airport station is a good place to buy your Swiss Pass.
Treat is as an "access all transport" pass (which is what it is). No procedures, no nothing!
Edit: Pauline has a typo in her link to the SBB (Swiss Federal Railways) website.
It should be: https://www.sbb.ch/en
"ch" is the abbreviation for Switzerland, not "cg", that is the Republic of Congo!