Huawei P20 Pro charging in USB host mode
I managed to achieve this (and use an HDMI monitor) by using a USB C Power Delivery hub, and a PD charger. The hub included as well an HDMI output, but it's cheap (around 30€). 
I think I used this one here.
I bought two different ones but the second one does not seem to be available anymore on Amazon and the link does not work anymore.
The charger I used is an USB C PD 41W charger from Auckly.