to Create the workitem in c# language

You will have to use the REST/OSLC API to do that. In that case the language you use does not matter. You can search this forum. There have been a lot of questions and answers. There is also an OSLC workshop in the library you can have a look at. It is a bit outdated. The best examples would likely be in the Java Code of the Eclipse Lyo samples that show examples of how to do the communication. Unfortunately the implementation hides a lot of details.

Some links I found: https://jazz.net/forum/questions/122146/visual-studio-api?page=1&focusedAnswerId=122147#122147

https://jazz.net/wiki/bin/view/Main/APIUsage#How_does_the_VS_Client_talk_to_t

https://jazz.net/wiki/bin/view/Deployment/CLMProductAPILanding has a link to Authentication of a native client with a Jazz-based application

I have some examples here: https://rsjazz.wordpress.com/2019/05/13/using-a-rest-client-to-authenticate-to-elm-clm-applications/