The entity field should contain the object's (in this case, DiscussionResponse) properties, and optionally the desired object id. Please check the REST API documentation for more information.
There are 2 ways to figure out Clarizen entity fields.
1. You can use the API Metadata services to get the list of fields and their metadata.
2. In your Clarizen account, browse to Settings -> Configure (you may want to choose the entity class on the left side) to see the class API name and the list of standard fields with their API name.
Hope this helps,