JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  <input type="text" name="foo">
  <button>Ок</button>
  <div id="bar" class="hidden">pew!</div>
</form>

CSS

.hidden {
  display: none;
}

JavaScript

const foo = document.querySelector('input[name=foo]')
const bar = document.getElementById('bar')
const secretPhrase = 'test'
foo.addEventListener('input', function(){
  if (this.value === secretPhrase) {
    bar.classList.remove('hidden')
  }
})