JSFiddle - React, Tailwind, and code Playground

HTML

<section class="container">
  <section class="login-form">
    <form method="post" action="/sistema/validaEmpresaCurriculo.php" role="login">
      <img src="imgs/logo.png" class="img-responsive" alt="" />
      <select class="form-control input-lg" name="tipoUsuario" id="tipoUsuario">
        <option value="0" selected disabled>Selecione</option>
        <option value="E">Empresa</option>
        <option value="C">Candidato</option>
      </select>
      <input type="number" id="username" name="username" class="form-control input-lg" placeholder="username" />
      <input type="password" id="password" name="password" class="form-control input-lg" placeholder="password" />
      <button type="submit" name="go" class="btn btn-lg btn-primary btn-block">Acessar</button>
      <div>
        <a href="/novasenha">Recuperar Senha</a>
      </div>
    </form>
    <div class="form-links">
      <a href="#">www.ondenaweb.com.br</a>
    </div>
  </section>
</section>

JavaScript

(function(){
	
  var $username = document.getElementById('username');
  
  document.getElementById('tipoUsuario').addEventListener('change', function(){
  	var placeholder = this.value == 'E' ?
    									'CNPJ' :
                      'CPF';
    
    $username.placeholder = placeholder;
  });
})();