HTC One M8 for Windows - Manual (Page 51)

Charging the battery
Before you turn on and start using HTC One, it is recommended that you
charge the battery.
Use only the adapter and the USB cable that came with HTC One
to charge the battery. When the battery power is too low, make
sure to use the power adapter to charge, not the USB cable
connection with your computer.
1. Insert the small end of the USB cable into the USB/Charging Port.
Your first week with your new phone 51