JSFiddle - React, Tailwind, and code Playground

HTML

<a id="formLink" href="javascript:void(0)">Обратная связь</a>
<div id="box">
    <div id="box-inner">
    </div>
  <form>
   <textarea placeholder="Ваш отзыв"></textarea>
   <button>Отправить</button>
  </form>
</div>

CSS

* {
    font-family: Arial;
}
#box {
    position: fixed;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    z-index: 999;
    display: none;
}
#box-inner {
    position: absolute;
    left: 0px;
    right: 0px;
    top: 0px;
    bottom: 0px;
    background: blue;
    opacity: 0.3;
    display: none;
}
#box form {
    position: absolute;
    top: 0; bottom: 0; right: 0; left: 0; /* центрирование */
    margin: auto;  /* центрирование */
    height: 10em;
    width: 20em;
    border: 1px solid black;
    background: white;
    padding: 5%;
}
textarea {
    width: 90%;
    height: 80%;
    border: 1px solid black;
    transition: 0.3s;
    outline: none;
}
textarea:focus {
    border: 1px solid blue;
}
button {
    padding: 1% 10%;
    background: white;
    border: 5px solid silver;
    transition: 0.5s;
    font-size: 1.05em
}
button:hover {
    border: 5px solid black;
}

JavaScript

document.getElementById("formLink").onclick = function()
{
    document.getElementById("box").style.display = "block";
    document.getElementById("box-inner").style.display = "block";
}
document.getElementById("box").onclick = function(e)
{
    if(e.toElement == document.getElementById("box-inner"))
    {
        document.getElementById("box").style.display = "none";
        document.getElementById("box-inner").style.display = "none";
    }
}