It should be noted that this comes with no power supply. You’ll have to farm out your own. Turns the deal into a stinker.
No way to use it with an hdmi source as is. Your only option is to drain the usb-c input device connected for power.
It should have come with a power cable.
If yours is missing, reach out to ViewSonic to see if they can replace it.
Monitors such as these are mainly targeted to folks wanting a portable secondary monitor – which means there may not be an AC outlet nearby. The power cable as noted is the USB C cable.
Per the user’s manual (downloaded from Viewsonic, this just needs a port that can deliver more than 10 watts. There are many options for that, such as these:
… or any number of options at the mothership too. If you have an existing USB power adapter (5V, 2A minimum), that’ll work too.
FWIW, a number of phones have not included power adapters for a few years now either – and they’re $1,000+.
(Note that I am not staff. I just volunteer to help out on the forums.)