JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="Red" id="myInput">
    <input type="text" value="Blue" id="myInput2">
    <input type="text" value="Black" id="myInput3">
    <input type="hidden" value="" id="output">
    <button onclick="myFunction()">Copy text</button>
<script>
    function myFunction() {
      var copyText = document.getElementById("myInput");
      var copyText2 = document.getElementById("myInput2");
      var copyText3 = document.getElementById("myInput3");
      var output = document.getElementById("output");
      output.value = copyText.value + "_" + copyText2.value + "_" + copyText3.value;
			output.select();
      document.execCommand("copy");

    }
</script>