STM has released a generic Serial to USB converter board named STEVAL-PCC018V1.
Below there is the typical wiring connections from the STEVAL-PCC018V1 to the UART of STM32.
Using the STEVAL-PCC018V1 is possible also power the user board because the STEVAL-PCC018V1 include also a regulator of 3,3V max 500 mA.
The max current is limited of the PC USB interface.