You need one with a screwplate at the botton, so you can just use any existing screw to bolt it on.

https://www.amazon.com/Goodway-Universal-Motorcycle-Waterproof-Rotation/dp/B07BRR9RPM/ref=sr_1_3?ie=UTF8&qid=1550102893&sr=8-3&keywords=phone+mount+motorcycle+screw

This chinesium mount has one, I also bought some cheap one and it hat two mounts, one to screwable and one normal U-shape.