JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/3.5.2/select2.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/3.5.2/select2.css">
<select>
    <optgroup label="Level_1">
        <optgroup label="Level_2">
            <option value="option_1">option_1</option>
            <option value="option_2">option_2</option>
        </optgroup>
        <optgroup label="Level_2">
            <option value="option_1">option_1</option>
            <option value="option_2">option_2</option>
        </optgroup>
    </optgroup>
    <optgroup label="Level_1">
        <optgroup label="Level_2">
            <option value="option_1">option_1</option>
            <option value="option_2">option_2</option>
        </optgroup>
        <optgroup label="Level_2">
            <option value="option_1">option_1</option>
            <option value="option_2">option_2</option>
        </optgroup>
    </optgroup>
</select>

CSS

select {width: 200px}

JavaScript

$('select').select2();