There is no discussion or forum source type in Bind Search API, so I think you cannot do this. As a workaround you can append some text to query as "bind community or forum" to narrow the result within the content you want. For more question you can contact
from the page by clicking "Get support for this offering":
Microsoft is conducting an online survey to understand your opinion of the Msdn Web site. If you choose to participate, the online survey will be presented to you when you leave the Msdn Web site.