A/F ratio install

I need to tie in the AF gauge, and was wondering which of the four wires was the sending wire.

The 02 sensor has four wires.
1 Black
2 White
1 Purple or lavenderish.

I am assuming its the purple wire I need to tie into, or is it the black.

Should I tie it in at the 02 sensor wire or the wires that are on the sensor extenders??

thanks for the help