To make your form work with Form Relay, change the action attribute value of your form to point to your relay and make sure the relay ID is correct. ID in this example is xxxx inside action form attribute

<form action="https://formrelay.com/r/xxxx" method="POST">
  <label for="name">Name</label>
  <input type="text" id="name" name="name">

  <label for="favorite_animal">Favorite animal</label>
  <select id="favorite_animal" name="favorite_animal">
    <option>Cat</option>
    <option>Dog</option>
    <option>Bear</option>
  </select>

  <label>Favorite food</label>
  <input type="checkbox" name="food_pizza" id="food_pizza" value="true">
  <label for="food_pizza">Pizza</label>
  <input type="checkbox" name="food_cheeseburger" id="food_cheeseburger" value="true">
  <label for="food_cheeseburger">Cheeseburger</label>
  <input type="checkbox" name="food_spinach" id="food_spinach" value="true">
  <label for="food_spinach">Spinach</label>

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