Windows store Lync app, URI help

We are developing a Windows Store app on the 8.1 framework and are trying to invoke the Windows Store Lync application by using URIs. This works great for the TEL URI since Lync is set up to listen for it. Unfortunately, it does not want to listen to anything with SIP or IM (as the desktop version does). 

I just need to know what URI and parameters are needed to make Lync open a new IM conversation.

Thanks!

Sean