[Solved] Popup text box in HTML with javascript

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Does this meet your requirements?

function showPopup() {
  document.getElementById('2').style.display = "block";
}

function syncValueWith2() {
  document.getElementById('2').value = document.getElementById('1').value;
}

function syncValueWith1() {
  document.getElementById('1').value = document.getElementById('2').value;
}
<textarea onkeyup="syncValueWith2()" id="1"></textarea>
<br>
<textarea onkeyup="syncValueWith1()" id="2" style="display: none;"></textarea>
<input type="button" value="Show Popup" onclick="showPopup()">

solved Popup text box in HTML with javascript