Here's the code I wrote:

#include "Keyboard.h"

const char FUNCTION_F14 = 0xF1;
int down = 0;

void setup() {
  // make pin 2 an input and turn on the
  // pullup resistor so it goes high unless
  // connected to ground:
  pinMode(2, INPUT_PULLUP);
  // initialize control over the keyboard:
  Keyboard.begin();
}

void loop() {
  int nowDown = digitalRead(2) == LOW;

  if (down != nowDown) {
    down = nowDown;

    if (down) {
      Keyboard.press(FUNCTION_F14);
    } else {
      Keyboard.release(FUNCTION_F14);
    }
  }
}

Note that the #include was meant to use angle brackets, but Lemmy ate them. If this doesn't work, change it back to angle brackets around the Keyboard.h instead of quotes.

Also, the parts I used:

• https://vi.aliexpress.com/item/1005003583458697.html
• https://vi.aliexpress.com/item/32846843498.html

I wired it up like in the photo, and just laid it on a bed of hot glue so the USB port sticks out the hole. I had intended to get a mini USB extension cable inside the pedal, but the one I ordered turned up defective, and this worked out just fine.