JSFiddle - React, Tailwind, and code Playground

HTML

<select id="sel">
    <option selected="selected" disabled="disabled" value="-1">Choose Below</option>
    <optgroup label="--------">
        <option value="0">A</option>
        <option value="1">B</option>
        <option value="2">C</option>
    </optgroup>
</select>

JavaScript

document.getElementById('sel')
    .addEventListener('change', function () {
        if (this.selectedIndex === 0) return;
        this.options[0].value = this.value;
        this.options[0].textContent = this.options[this.selectedIndex].textContent;
        this.selectedIndex = 0;
    }, false);