JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="input-to-replace">

CSS

#input-to-replace{
    opacity: 0.2;
    /*you want display: none; or something else here, the opacity is 
    for you to see how it stays in sync*/
}
textarea{
    display: block;
    width: 300px;
    height: 300px;
}

JavaScript

var inputToReplace = document.getElementById('input-to-replace');
var replacementTextarea = document.createElement("textarea");
document.body.appendChild(replacementTextarea);

var copyValueOver = function(){
    // You may have to do some fancy encoding of newlines and other nasty bits, depending on what you expect to get.
    inputToReplace.value = replacementTextarea.value;
};

// This bit may be quite tricky depending on where the data comes from and so on, change and keyup are easy and just to show the way
replacementTextarea.addEventListener('change', copyValueOver);
replacementTextarea.addEventListener('keyup', copyValueOver);