Just init in the browser console and spam what you want

twitch-spam.js

class Spammer {
  chatInput = null;
  sendButton = null;
  valueSetter = null;

  constructor() {
    this.chatInput = document.querySelector('[data-a-target="chat-input"]');
    this.sendButton = document.querySelector('[data-a-target="chat-send-button"]');
    this._setupValueSetter();
  }

  /*
   * Some browsers don't support private methods,
   * replace this with `#setupValueSetter()` in the future
   */
  _setupValueSetter() {
    const valueSetter = Object.getOwnPropertyDescriptor(this.chatInput, 'value').set;
    const proto = Object.getPrototypeOf(this.chatInput);
    const prototypeValueSetter = Object.getOwnPropertyDescriptor(proto, 'value').set;

    if (valueSetter && valueSetter !== prototypeValueSetter) {
      this.valueSetter = prototypeValueSetter;
    } else {
      this.valueSetter = valueSetter;
    }
  }

  send(value, numberOfTimes = 1) {
    for (let i = 0; i < numberOfTimes; i++) {
      this.valueSetter.call(this.chatInput, value);
      this.chatInput.dispatchEvent(new Event('input', { bubbles: true }));
      this.sendButton.click();
    }
  }
}

/* USAGE:

const spammer = new Spammer();
spammer.send('😎'); // to send one time
spammer.send('😎', 10); // to send 10 times

*/