We have a request from a customer who wants to integrate their application with RC. In fact, they want to make a request from their application and the approval process done in RC. Their application uses webservices.
Does anyone know how to do this? Is there any documents available which has a step by step approach on how to achieve this?
Are you still on RC2007? If so, see the RC2007.1 ServiceLink Requisition API User Guide. This, however, is older functionality.
There has been an update to web services functionality starting in 2008.2 (see nS FOS2008.2 Web Services User Guide), which makes interacting with RequestCenter via web services far easier. You might want to consider upgrading (and not just to 2008 but to the latest version which is NS9.1).