Hello Gagan,
It should be possible to determine which of the OpenRTB fields you need to provide by determining what it maps to (in this case, html_snippet) and verifying whether that is a required field for the DoubleClick rtb protocol. Ultimately, the OpenRTB response will be converted into the DoubleClick protocol, and when it does, it will need to have the required fields for that protocol. In the response, you should provide the html_snippet (or video_url / native_ad).
Regards,
Mark