JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="username" placeholder="Enter your name" />
<button>Start Game</button>

JavaScript

const usernameInputField = document.querySelector('#username'),
  button = document.querySelector('button');

button.addEventListener('click', () => {
  if (usernameInputField.value != "") {
    localStorage.setItem('user', usernameInputField.value);
  }
  alert('Username Saved Successfully. Starting the game...');
})