fireAnEvent.html

<template>
  <button onclick={handleClick}>Click me</button>
</template>

<script>
import { LightningElement } from 'lwc';

export default class EventFiringComponent extends LightningElement {
    handleClick() {
        const event = new CustomEvent('myevent', { detail: { message: 'Hello World' } });
        this.dispatchEvent(event);
    }
}
</script>