Diagrammatically, the headlight has three posts in a row with a filament each connected from center to the outer ones. Two filaments, Hi/Lo.
The center is ground, green, or battery minus. Apply battery plus to either of the outer connections and a filament glows. One of them is high watts the other is low watts. The High/Low switch routinely handles this power routing.
The design of some switches will ground unused contacts when not needed for the circuit. I don't have a 400F switch to check the design of that particular switch. However, if it IS designed to ground unused terminals and you interupt the ground lead with a separate switch. Applying power to one of the outer posts will find a ground at the opposite post through BOTH filaments. The voltage will be divided by each filament and will operate at reduced power accordingly, I.E. they will be very dim, but they will still draw power.
If the Hi/Lo switch doesn't ground unused contacts then a single pole single throw switch can be employed in the ground path to douse the headlight regardless of Hi/Lo switch position.
However, if you leave the ground path intact, place your single pole switch in either the White or blue lead path at the headlight, it will douse the headlight whenever the lead is selected by the Hi/Lo switch.
If you can't be bothered to manually set two switches to douse the headlight. Then either install a DPST switch to interrupt both the white and blue leads at the headlight, OR, run two wires from your SPST switch down to the junction area under your fuel tank and insert the switch between the Black/red male and female bullet connectors.
I'm done, now...