将部分填写的表单发布到webhook并在用户返回时保存字段

I have the following form that posts to a webhook on submission. However i need to post the fields to the webhook even if the user does not submit the form in order to capture form abandonments. Additionally if they return back to the form i wanted to have the previously filled inputs fill in. Wondering if someone can help me with this?

Most important is that it posts the form to a webhook even if they don't submit the form.

    <form id="signupForm" 
action="https://hooks.zapier.com/hooks/catch/298145/cmmltf/" 
method="post" class="uk-grid-small" uk-grid>

<div class="uk-width-1-2@s">

    <input id="fname" name="firstName" class="uk-input" type="text" 
placeholder="First Name">
</div>
<div class="uk-width-1-2@s">`enter code here`
    <input name="lastName" class="uk-input" type="text" 
placeholder="Last Name">
</div>

 <div class="uk-width-1-1">
    <input name="email" class="uk-input" type="email" 
 placeholder="Email">
 </div>
 <div class="uk-width-1-1">
    <input name="password" class="uk-input" type="password" 
 placeholder="Password">
 </div>

  <div>
 <button class="uk-button uk-button-default" input type="submit" 
 value="Submit" id="submitBtn">Submit
  </div>
 </form>