JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://cdn.jsdelivr.net/semantic-ui/2.2.9/components/form.min.css">
<div class="ui form">
  <div class="ui field">
    <select name="mails" id="mails">
      <option value="0">Choisissez un mail..</option>
      <option value="1">Okéa - Votre livraison est en cours d'acheminement !</option>
      <option value="2">Fakebook - Vous avez un message de Jean Paindemie.</option>
      <option value="3">EasyCandy - 50% de réduction sur vos produits favoris !</option>
      <option value="4">Stim - Un article de votre liste de souhaits est en soldes.</option>
    </select>
   </div>
   <div class="ui field">
     <textarea name="mail" id="mail" readonly></textarea>
   </div>
</div>

JavaScript

let ajax = function (url) {
  return new Promise(function (resolve, reject) {
    let req = new XMLHttpRequest()
    req.open('GET', url, true)
    req.onreadystatechange = function (aEvt) {
      if (req.readyState == 4) {
         if(req.status == 200)
           resolve(req.responseText)
         else
           reject(req)
      }
    };
    req.send(null)
  })
}

document.querySelector('#mails').addEventListener('change', function (e) {
    e.preventDefault();
    var choice = this.value, textarea = document.querySelector('#mail');
    ajax('https://jsonplaceholder.typicode.com/posts/' + choice).then(response => {
        var result = JSON.parse(response)
        textarea.textContent = result.body;
    })
});