Arduino ESP8266: Connecting external power supply and USB from PC at the same time
According to the schematics of the board, there is a diode (D1) between the VDD5V and the VDDUSB net to prevent any current flowing back, towards the laptop's USB port.



Just make sure that you connect your external power supply's GND to your board's GND, and the 3.3 V to the board's 3.3 V pin.