JSFiddle - React, Tailwind, and code Playground

HTML

<label>Price Start</label>
							  <input type="number" class="form-control" placeholder="Total Harga Diberikan" required="required" name="price" onkeyup="calculate()" id="price" />
<br />
<label>Discount</label>
							  <input type="number" class="form-control" placeholder="Nominal Potongan" name="jmlvoucher" onkeyup="calculate()" id="jmlvoucher" />
<br />
<label>Total</label>
							  <input type="number" class="form-control" placeholder="Total yang harus dibayarkan" required="required" name="total" id="total" />

JavaScript

function calculate(){
		var num1 = document.getElementById("price").value;
		var num2 = document.getElementById("jmlvoucher").value;

		var sum=parseFloat(num1)-parseFloat(num2);
		document.getElementById('total').value=sum.toString();
	}