<input id="input" style="width:200px" value="Select here and click the button">
<button id="button">Wrap selection in stars *...*</button>

<script>
button.onclick = () => {
  if (input.selectionStart == input.selectionEnd) {
    return; // nothing is selected
  }

  let selected = input.value.slice(input.selectionStart, input.selectionEnd);
  input.setRangeText(`*${selected}*`);
};
</script>