The Spektrum adapter provides regulated 3.3V from unregulated (up to 15V) ESC/BEC power supplied via Servo headers to the board.
Please remember that USB does not provide power to the FlexiPort or MainPort.
|GND||1 - Black||2 - Black|
|PWR||+5V to adapter||1 - Orange (+3.3V)|
|TX||3 - Blue||-|
|RX||4 - Orange||3 - Grey|
Schematic is pretty simple, you can made your adapter with a few components: Like here