JSFiddle - React, Tailwind, and code Playground

HTML

<select multiple="multiple" size="10" id="selection">  
    <option value="test1">Test1</option> 
    <option value="test2">Test2</option>
</select>
<button id="btn" onclick="copyToClipboard()">Testing</button>

JavaScript

function copyToClipboard() {
    var selectBox = document.getElementById("selection");
    var text = "";
    for (i = 0; i < selectBox.length; i++) {
        text += selectBox.options[i].value + ",";
    }
  window.prompt("Copy to clipboard: Ctrl+C, Enter", text);
}