[Dilettante_Pro]

<div id="todo_root">
  
    <ul id="todo">
        <li><input type="text" id="input"><button id="submit">Добавить</button></li>
    </ul>
 
</div>
<script>
function createItem() {
    var li=document.createElement('li');
    li.innerText=element.value;
    var ul=document.getElementById('todo');
    ul.appendChild(li);
}
var element =document.getElementById("input");
document.getElementById("submit").onclick=createItem;
console.log(element);
</script>

Или так

<div id="todo_root">
    <form id="form">
    <ul id="todo">
        <li><input type="text" id="input"><button id="submit">Добавить</button></li>
    </ul>
    </form>
</div>
<script>
function createItem() {
    var li=document.createElement('li');
    li.innerText=element.value;
    var ul=document.getElementById('todo');
    ul.appendChild(li);
}
var element =document.getElementById("input");
document.getElementById("submit").onclick=createItem;
document.getElementById("form").onsubmit = function() { return false;};
console.log(element);
</script>