JSFiddle - React, Tailwind, and code Playground

HTML

<form id="contact" action="" method="post">
<div>
<input type="text" name="firstname" id="firstname">
</div>
    <div>
    <input type="text" name="lastname" id="lastname">
    </div>
    <div>
    <input type="text" name="age" id="age">
    </div>
    <div>
     <input type="hidden" name="refer_page" id="refer_page" value="success.php">
     </div>
     <div>
     <input class="control--custom-input" type="radio" value="true" id="MedicalExpenseYes" name="MedicalExpense">Yes
     </div>
     <div>
     <input class="control--custom-input" type="radio" value="false" id="MedicalExpenseNo" name="MedicalExpense">No
     </div>
     <div>
     <select id="cs-choice" name="Profession" class="form-input form-input--select">
                          <option value="--">Select title</option>
                          <option value="nurse">Nurse</option>
                          <option value="teacher">Teacher</option>
                          <option value="public_servant">Public Servant</option>
                          <option value="other">Other</option>
                        </select>
                        </div>
    <input type="submit" value="submit">
</form>

JavaScript

let theForm =  document.getElementById("contact");
theForm.onsubmit = function(event) {
    event.preventDefault();
    
    let rawData = new FormData(theForm);
    let data = {};
    
   for(let pair of rawData.entries()) {
     data[pair[0]] = pair[1];
    }
    let contactData = JSON.stringify(data);
    console.warn(contactData);
};