How do I ask for onsite job opportunities in TCS being in a project?

First thing, your project should have on-site requirement. Some projects are based in Indian so there is no chance of on-site there.

Secondly it depends on how many people are smarter than you. They will get on-site opportunities before you.

Sometimes it happenes that those people are married or previously have been on on-site duty due to which they might reject on-site offer and then you can get one.

Also you can ask your manager regarding on-site scope for your particular project.

Try Resigning.

I have completed 2.5 years in the company