>>1871371this is a common joke amongst fixietards
you modify a downtube shifter. There are clamp on ones if you don't have bosses. Stem shifters might also be good as they have built in cable stops. You might also clamp on a cable stop which can be done simply by taking a barrell adjuster and hose clamping it on the frame somewhere
I can kind of see the point in a semi-functional sketchy brake that requires a hand off the bars to scrub speed and as a laugh but if you're gonna be cluttering up your bike i mean come on why not just run real brakes?