Redirect URL after sending envelope

I was looking at the release notes and saw support for postSendOption.redirectURL in the POST /agreements/{agreementId}/ endpoint.
I don't understand how to use this though. I've tried passing the param as {postSendOption: {redirectUrl: <myUrl>}} to POST /agreements but that doesn't seem to work.
Its also worth nothing that the REST API v6 documentation (https://secure.na1.adobesign.com/public/docs/restapi/v6#://) makes no mention of this parameter.
Also the only endpoints listed in the REST API docs are POST /agreements and PUT /agreements/{agreementId}.
What am I missing?
