There is nothing special about configuring a virtual switch in RM. They are treated the same as any other switch. I think maybe your question is more “how to configure a virtual switch for use in RM”? It’s a good question, and can be confusing, since ST changed the terminology somewhere along the line. What was previously known as “virtual” now appears in the IDE as “simulated.” Further, there is also “Momentary Button Tile” and “On/Off Button Tile”… both of which are simulated/virtual.
You add a new virtual switch from the IDE at My Devices > +New Device, then select the appropriate device from the scrolling ‘Type’ list. Note that not everything there is a “virtual” device, but all the ones I mentioned earlier are. If all you need is a binary on/off function for controlling with RM, then you probably want “On/Off Button Tile.” Full instructions for adding a virtual device can be found here (it’s for a virtual dimmer, but the procedure is identical).