Az far as I can tell, you would need to configure the D12 pin to output. If you need to switch on the FTU you would need to set the pin to digital HIGH and to switch off the FTU you need to set the pin to digital LOW.
in the setup function of the Arduino sketch add:
pinMode( 12, OUTPUT);
digitalWrite ( 12, LOW); // FTU OFF
to switch on FTU:
digitalWrite( 12, HIGH);
It is important that the D12 digital I/O pin is also the MISO line of the SPI peripheral, so if you planned to use the SPI you should change this pin to an unused I/O pin.
You should also consider to switch off the FTU after successful separation, so the Nichrome Wire wouldn't make any damage in the payload. You should determine the cut-off time (with margin) or add a feedback sensor to confirm the separation.