HTMLSelectElement.form

<form id="pet-form">
  <label for="pet-select">Choose a pet</label>
  <select name="pets" id="pet-select">
    <option value="dog">Dog</option>
    <option value="cat">Cat</option>
    <option value="parrot">Parrot</option>
  </select>

  <button type="submit">Submit</button>
</form>

<label for="lunch-select">Choose your lunch</label>
<select name="lunch" id="lunch-select">
  <option value="salad">Salad</option>
  <option value="sandwich">Sandwich</option>
</select>

<script>
  const petSelect = document.getElementById("pet-select");
  const petForm = petSelect.form; // <form id="pet-form">

  const lunchSelect = document.getElementById("lunch-select");
  const lunchForm = lunchSelect.form; // null
</script>