I've done this a few times, it is possible and yes you just show up at the gate same as a domestic flight. Even for a flight to Brazil where they have to check passports/visas, it was the same process at the gate in ORD: "Hi, I'm with [carrier] and I'd like to try and jumpseat to São Paulo, please".

But, like previous posters said, while you can "jumpseat" (as in, show up and ask to ride space available in the back) on international flights, you cannot occupy the actual jumpseat in the cockpit (yet).

Before I worked for United, I jumpseated on United to/from the UK, Canada, Brazil, and Italy. One thing to remember is that coming back in to the US, you'll have to pay the taxes/fees in order to jumpseat (this is pricy from LHR particularly, but was about $40 from Vancouver). So for the return leg, I recommend going to the ticket counter to ask about this. Also, agents in other countries often associate the word "jumpseat" with sitting in the actual seat and will then tell you it is not allowed. In explaining exactly what you want to do, it used to help to use the term "OMC" instead as that's their term for what you want to do. I'm not sure if this term is still in use as I know the software changed after the merger.

Good luck and hope you get on.