The method RC6 is wrapping here is ICoreWebView2 -> Sub CallDevToolsProtocolMethod(ByVal methodName As LongPtr, ByVal parametersAsJson As LongPtr, ByVal handler As ICoreWebView2CallDevToolsProtocolMethodCompletedHandler)

The results are returned to your implementation of ICoreWebView2CallDevToolsProtocolMethodCompletedHandler. RC6 does not look to have a wrapper for that, though I haven't looked too thoroughly-- see if a name like that exists in the events.

Failing that, see if RC6 allows you to get a pointer to its internal ICoreWebView2 interface; if you can get such a pointer, you can use oleexp.tlb for the interfaces to invoke the command manually to your own implementation of the completion handler.