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. 
| | FlexiPort MainPort | Spektrum |
|---|
| GND | 1 - Black | 2 - Black | | PWR( Note1) | +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 | 
