add.js

function addNumbers() {
  var firstNumber = document.getElementById("firstNumber").value;
  var secondNumber = document.getElementById("secondNumber").value;
  var sum = firstNumber + secondNumber;
  document.getElementById("sum").innerHTML = sum;
}

index.html

<form>
  <label for="First Number">First Number:</label>
  <input id="firstNumber" type="text"><br>
  <br>
  <label for="Second Number">Second Number:</label>
  <input id="secondNumber" type="text"><br>
  <br>
  <label for="Answer">Answer</label>
  <input id="sum" type="text"><br>
  <br>
  <button onclick="addNumbers();">Submit</button>
</form>