JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1" onsubmit="return false;">
    <select name="leftList">
        <option value="1">Left List Option 1</option>
        <option value="2">Left List Option 2</option>
    </select>
    <select name="rightList">
        <option value="1">Right List Option 1</option>
        <option value="2">Right List Option 2</option>
    </select>
    <button onclick="moveRight()">Move option to the right</button>
</form>

JavaScript

function moveRight() {
    var selItem = document.forms[0].leftList.selectedIndex;
    if (selItem == -1) {
        window.alert("You must first select an item on the left side.")
    } else {
        document.forms[0].rightList.add(document.forms[0].leftList[selItem], null);
    }
}