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.

Schematic is pretty simple, you can made your adapter with a few components: Like here

You can get the 3.3V the satellite needs from somewhere else on the board, directly from regulator or more easy with a wire soldered to one of SBL pads that provides +3.3V.

You must bind your satellite receiver by connecting it to the FlexiPort first, but once it is bound you can move it to the Main Port.

Enable binding mode

1. Connect your satellite receiver to the FlexiPort.
2. Connect a USB cable to the board without any other power connected.
3. Go into Configuration tab > Hardware tab, set both Receiver port and MainPort to Disabled (assuming that nothing is connected to either).
4. Set FlexiPort to DSM.
5. Leave Telemetry speed and GCS speed at their default settings (57600).
6. Click Save.
7. Disconnect the USB cable and wait 30 seconds for a complete hardware reset.
8. Reconnect the USB cable.
9. Go into System tab > UAVObject Browser, go to Settings > HWSettings > DSMxBind.
10. Set DSMxBind to the DSMxBind value used for your receiver (usually 5 (DMS2) or 9 (DSMX)).
11. Click Save
12. Disconnect the USB cable.

Bind receiver

1. Connect your satellite receiver to the FlexiPort.
2. With the USB cable disconnected, apply power to the board (via servo headers). The LED on the satellite flashes rapidly, indicating that it is in bind mode.
3. Power on your transmitter in its bind mode. (See the transmitter manual for the details of how to activate binding.)
4. When binding is complete, the satellite LED blinks slowly, then lights continuously.

Disable binding mode

1. Connect the USB cable.
2. Go into System tab > UAVObject Browser and set DSMxBind to 0. (This turns off binding mode.)
3. Click Save.