I am using this one: USB Type-C to HDMI Adapter - 4K | USB-C Video Adapters | StarTech.com on my Lenovo T470 (Work) notebook.
Normally this port is used by the power adapter, but I've kept the old power adapter from the Lenovo T450 so that the USB-C port was free for a second screen (Third if you count the notebook screen).
I suspect there might be some proprietary handshake thing going on with the Lenovo adaptor.
I'll get something else to test tomorrow.
the important thing for us is to use the USB-c/thunderbolt connector as "primary" display, so that it is possible to boot into BIOS using only that display port.