<form id="form" action="https://dummy.restapiexample.com/api/v1/employee/1">
  <input type="text" placeholder="Телефон" value="123" required>
  <button type="submit">Отправить</button>
</form>

const form = document.getElementById('form');

form.onsubmit = (e) => {
  e.preventDefault();
  
  const url = form.action;
  const formData = new FormData(form);
  const xhr = new XMLHttpRequest();
  
  xhr.onreadystatechange = () => {
    if(xhr.readyState === 4) {
      const result = JSON.parse(xhr.responseText);
      console.log(result);
      alert(result.employee_name);
    }
  }
  
  xhr.open("GET", url); //Метод и URL запроса
  xhr.send(formData);
};