I think this is the code needed in the section commented by "Jessica", I mean the part in the comment "Código para adicionar en la lista de Actividades"
<!DOCTYPE html>
<html lang="es">
<head>
<meta charset="UTF-8">
<title>Appending a Li to an Ul</title>
</head>
<body>
<ul class="Actividades">
<li>Ir al médico</li>
<li>Cambiar la contraseña del Wi Fi</li>
<li>Estudiar Javascript</li>
</ul>
<form class="adicionar-tarea">
<input type="text" name="tarea">
<button id="boton-adicionar">Guardar</button>
</form>
</body>
<script>
var form = document.querySelector('.adicionar-tarea');
var boton = document.querySelector('#boton-adicionar');
boton.addEventListener("click", function (event) {
event.preventDefault()
console.log(form.tarea.value);
var ul = document.querySelector(".Actividades");
var li = document.createElement("li");
li.appendChild(document.createTextNode("New Item in Unordered list"));
ul.appendChild(li);
});
</script>
</html>
