Here is how i connected the button to the unit.

I've managed to read button click from M5Unit 8Servos with this code.
I'm using an M5Stack AtomS3 Lite.

#include <Arduino.h>
#include <M5Unified.h>
#include <Wire.h>
#include <M5_UNIT_8SERVO.h>

#define SERVO_PIN 0
#define START_BUTTON_PIN 1

// put variables here:
M5_UNIT_8SERVO unit_8servo;
bool btnState;
unsigned long btnClieckedMs;

// put function declarations here:

void setup() {
  M5.begin();
  Serial.begin(115200);

  Serial.println("### Setup - start");

  while (!unit_8servo.begin(&Wire, G2, G1, M5_UNIT_8SERVO_DEFAULT_ADDR)) {
    Serial.println("extio Connect Error");
    delay(100);
  }

  //unit_8servo.setAllPinMode(SERVO_CTL_MODE);
  unit_8servo.setOnePinMode(SERVO_PIN, SERVO_CTL_MODE);
  unit_8servo.setOnePinMode(START_BUTTON_PIN, DIGITAL_INPUT_MODE);
  
  btnState = false;

  Serial.println("### Setup - end");
}

void loop() {
  bool prevBtnState = btnState;                               // save last state for comparison
  btnState = unit_8servo.getDigitalInput(START_BUTTON_PIN);   // read new state

  if(prevBtnState == false && btnState == true) {             // if state has changed
    btnClieckedMs = millis();                                 // save millis
  } else if(prevBtnState == true && btnState == false) {      // if state has changed again
    if(millis() - btnClieckedMs > 50) {                       // check how long it took from 'false' to 'true' (debounce)
      Serial.print("Button clicked ");                        // do some fancy things when button clicked 
      Serial.println(btnClieckedMs);                      
    }
  }
}

I think i should pull out my multimeter and check what happens, if i configure the port as input.
Maybe I'll find out, how it's working.

The M5buttons schematics shows how to debounce the button input. Thank you very much for this picture.

My push button is a cheap metal 1NO push button from aliexpress
https://de.aliexpress.com/item/4000962003511.html?spm=a2g0o.productlist.main.29.28681ZX81ZX8eV&algo_pvid=980580c4-cbbf-4562-8505-6208e49f88f1&algo_exp_id=980580c4-cbbf-4562-8505-6208e49f88f1-14&pdp_npi=4%40dis!CHF!1.57!1.57!!!1.68!1.68!%402103010b17167024952887064e1da1!10000012730074410!sea!CH!2358164638!&curPageLogUid=D8W0ybIwBZDH&utparam-url=scene%3Asearch|query_from%3A

My idea is to connect it to one port of the M5Unit 8Servos.

The example on github is here.
https://github.com/m5stack/M5Unit-8Servo/blob/main/examples/DIGITAL_INPUT_OUTPUT/DIGITAL_INPUT_OUTPUT.ino

I don't know if i have to connect the push button between S and G on the 8Servos Unit or between V and S. And i'm wondering if i have to debounce the push button in hardware with an RC filter or it's already done in the unit. Or can i debounce the button in my arduino code?