Hi Vasilios Tzanoudakis,
In order to send a specific context from OpenSIPS to CGRateS you will need to set a custom context in the config for OpenSIPS.
The variable you are going to use is "Context", and the final line in the config is supposed to look like this : $cgr(Context) = "your_context_here".
This will populate the event with your desired context.
After doing this, we would appreciate if you could post the captured ngrep for us to analyze.
Thanks,
Andrei