WASAPI to Submix?

If you’re not familiar with it, WASAPI is the Windows Audio Session API.
It has a loopback mode that allows clients to record whatever sound is playing on the PC.

I would like to send this into FMixerDevice::GetMagnitudesForFrequencies (which uses the new FSpectrumAnalyzer)
If you don’t pass a submix to this function, it uses the master submix.

So the question is…how can I take my recording (a float buffer) from WASAPI and turn it into a submix?