Is the main bulb on the new one an H4? Then you would wire the 3 prong GSF connector to that. By "two-socket" did you mean two prong?
If that is to be on only when the high beam is on the wire it in parallel with the 3 prong high beam. If I'm reading the diagram white that would be tapping off the BW (Black/White) wire which is the common and the Y (Yellow) wire which is the 12V to the high beam. Make sure your fuse will handle the extra current of the parallel light.
If that is to be in parallel with the low beam then tap off the BW and W (white) wires.
I haven't done this and it's just a best guess.