JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form1" method="POST">
		<table style="border:1px solid black">
			<tr>
				<td>First Number</td>
				<td><input type="text" id="first"></td>
			</tr>
			<tr>
				<td>Second Number</td>
				<td><input type="text" id="second"></td>
			</tr>
			<tr>
				<td>Result</td>
				<td><input type="text" id="result"></td>
			</tr>
			
			<td>
				<button id="btn" value="Add" onClick="addNumbers(); return false;">Add</button>
			</td>
			
		</table>
	</form>

JavaScript

function addNumbers() {
			var firstNumber = document.getElementById('first').value;
			var secondNumber = document.getElementById('second').value;

			document.getElementById('result').value = firstNumber + secondNumber;

		}