kbdswitch/kbdswitch.ino

#define LED_PIN 1     // LED inside the pushbutton.
#define RELAY_PIN 3   // Pin, the relay (transistor) is connected to.
#define SWITCH_PIN 2  // The actual pushbutton.

bool relay_on = 0;
uint16_t debounce_time = 450;   // In ms. Adjust, if desired.

void setup() {
  pinMode(LED_PIN, OUTPUT);  
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(SWITCH_PIN, INPUT_PULLUP);  // Saves some space on the PCB.
}

void loop() {
  if (digitalRead(SWITCH_PIN) == LOW) {
    relay_on ^= 1;  // Toggle the relay state.
    digitalWrite(RELAY_PIN, relay_on);
    digitalWrite(LED_PIN, relay_on);  // One could switch the LED state around if desired.
    delay(debounce_time); // Simple switch debouncing.
  }
}