<form id="form">
  <input id="name" placeholder="Иван Иванов">
  <input id="phone" placeholder="+7 900 000-00-00">
  <button>Отправить</button>
</form>

<script>
document.getElementById("form").addEventListener("submit", async (event) => {
  event.preventDefault();

  const response = await fetch("https://api.formtomail.ru/send", {
    method: "POST",
    headers: {
      "Authorization": "Bearer YOUR_API_KEY",
      "Content-Type": "application/json"
    },
    body: JSON.stringify({
      body: {
        "Имя": document.getElementById("name").value,
        "Телефон": document.getElementById("phone").value
      }
    })
  });

  console.log(await response.json());
});
</script>