JSFiddle - React, Tailwind, and code Playground

HTML

<select id="dropdown">
    <option value="">Select an option …</option>
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="30">Three</option>
</select>

<input type="text" id="value" />

JavaScript

document.getElementById("dropdown").onchange = function() {
    document.getElementById("value").value = this.options[this.selectedIndex].value;
}