JSFiddle - React, Tailwind, and code Playground

HTML

<select id="pref">
    <option value="選択">選択</option>
    <option value="北海道">北海道</option>
</select>

<button type="button" id="test">テスト</button>

JavaScript

document.getElementById('test').onclick = () => {
	document.getElementById( "pref" ).value = '北海道';
}