How to pass a variable as an argument to ROS2 service?

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Hello everyone,

I would like to know if it's possible to an additional argument to a ROS2 service. The example that I follow only uses request and response as the function arguments.

https://docs.ros.org/en/foxy/Tutorial...

Here in the above example, the service function is defined as:

void add(const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request> request,
          std::shared_ptr<example_interfaces::srv::AddTwoInts::Response> response)

In my case, I want to pass an additional argument to that service function, a variable that is defined within the main(). Is it possible? If not, what's it the correct way of doing so?

PS: I am trying to implement my Python services in C++ therefore if what I am trying to do is more feasible in another way please feel free to help out.

Thanks!