JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
<head>
  <title>Test</title>
</head>
<body>
  <div id="conteneur">
    <div id="champ1">
      <label for="contrat">Contrat *:</label>
      <select id="contrat" name="contrat">
        <option value="CDI" selected="selected">CDI</option>
        <option value="CDD">CDD</option>
        <option value="Interimaire">Interimaire</option>
        <option value="Prestataire">Prestataire</option>
        <option value="Auxiliaire">Auxiliaire saisonnier</option>
        <option value="Stagiaire">Stagiaire</option>
        <option value="Alternant">Alternant</option>
      </select>
    </div>
    <div id="champ2">
    </div>
  </div>
</body>
</html>

JavaScript

document.getElementById('contrat').onchange = function(){
  if (this.value == 'Prestataire') {
	var new_input    =  document.createElement('input');  
	new_input.type   =  "text";
    new_input.id     =  'id_input';
	new_input.value  =  this.value;
	document.getElementById('champ2').appendChild(new_input);
  }
};