DNG 6.0 : How to retrieve Custom attribute value from requirement using OSLC.

I am using OSLC to create and retrieve requirement from DNG. I have two custom attributes and want to add their value of custom attributes at the time of  creating requirements. I ma using createResource method for creating requirement.
client.createResource(requirementFactory, requirement, OslcMediaType.APPLICATION_RDF_XML, OslcMediaType.APPLICATION_RDF_XML);

Can anyone help me how can i add and retrieve custom attribute value using same or other approach. Thanking in Advance.