The DN126M2 decoder is a Series 6, two function decoder. The decoder already has a Surface Mounted LED configured as the Forward Headlight. This LED is connected to the White (F0F) function. There are additional solder pads which will allow you to add a function for the Yellow (F0R) Rear headlight or it can be used to add another function such as a Beacon or Strobe. The functions can be programmed to suite your particular needs.
Remember to add the appropriate current limiting resistor for the lamp or LED that you're using.