JSFiddle - React, Tailwind, and code Playground

HTML

<select id="vehichleDropdown" onchange="onVehichleDropdownChange()">
  <option id="1">Car</option>
  <option id="2">Bike</option>
</select>

<br />
<br />
<select id="makeDropdown" onchange="onMakeDropdownChange()">
  <option id="1">Ford</option>
  <option id="2">Toyota</option>
</select>

<br />
<br /> Result
<input type="text" id="result" />

JavaScript

onVehichleDropdownChange = function() {
  var vehichleDropdown = document.getElementById("vehichleDropdown");
  var makeDropdown = document.getElementById("makeDropdown");
  makeDropdown.options[0] = null;
  makeDropdown.options[0] = null;

  if (vehichleDropdown.selectedIndex == 0) {
    var option1 = document.createElement("option");
    option1.text = option1.value = "Ford";
    makeDropdown.add(option1, 0);

    var option2 = document.createElement("option");
    option2.text = option2.value = "Toyota";
    makeDropdown.add(option2, 0);
  } else if (vehichleDropdown.selectedIndex == 1) {
    var option1 = document.createElement("option");
    option1.text = option1.value = "Harley";
    makeDropdown.add(option1, 0);

    var option2 = document.createElement("option");
    option2.text = option2.value = "Ducati";
    makeDropdown.add(option2, 0);
  }
  makeDropdown.onchange();
}

onMakeDropdownChange = function() {
  var resultText = document.getElementById("result");
  resultText.value = vehichleDropdown.options[vehichleDropdown.selectedIndex].text + ':' + makeDropdown.options[makeDropdown.selectedIndex].text;
}