Dynamic Registration Form submission using vanilla Javascript ajax

I want to use the ajax capability of not refreshing the page or rerouting the user to the registration page. Login / Register dialogue modals have been created. I have the login javascript ajax working perfectly using an ajax call to the login api without any configurations or added modules. I want to integrate the registration form in the same way. I would like the registration to:

1. show in a modal - this works.
2. Submit javascript code works with preventDefault
3. response is failing.

Ajax Post is returning '<' obviously this is returning some html. How do I know how to pass the data in the ajax call and what to expect as a response? My example below:

<style>
.scroll {
  //background-color: #fed9ff;
  //width: 600px;
  height: 500px;
  overflow-x: hidden;
  overflow-y: auto;
  //text-align: center;
  //padding: 20px;
}


.close { 
  margin: 10px 25px 0px;
}
#log-title { 
  margin-left:25px;
}
</style>

<a data-target="#register-modal" data-toggle="modal" class="" id="" href="#register-modal">Register / Create an Account </a>
 
<div id="overlay_container"></div>

 
<!--  Register to download button so stays on same page.  -->
<div id="register-modal" class="modal" tabindex="-1" role="dialog" aria-hidden="true">
  <button type="button" class="close" data-dismiss="modal" aria-label="Close">
    <span aria-hidden="true">&times;</span>
  </button> 
  <h3 id="log-title">Please register:</h3>   
  <div class="modal-dialog">
    <div class="modal-content scroll">
      <?php
        $register_form = \Drupal::service('entity.form_builder')->getForm(\Drupal\user\Entity\User::create([]), 'register', []);    
        // Load the service renderer
        $render = Drupal::service('renderer');
        $rform = $render->renderPlain($register_form);
        echo $rform;
      ?>
    </div> <!-- Register modal content end-->
  </div><!-- Register modal-dialog end -->
</div><!-- Register modal end -->

<script>
  var registerForm = document.getElementById('user-register-form');
  if (registerForm ) {
    // capture registration submit, prevent from submission and use ajax to get response
    registerForm.onsubmit = async (e) => {
      e.preventDefault();
        // callRegCaptcha();
      elms =document.getElementById("user-register-form").elements; 
      var url = "https://website/user/register"; //website is a true url

      fetch(url, {
        method: 'POST',
        body: elms
      })
      .then(response=>response.json())
      .then(json => {
        console.log(json);
      });
    }
  }
</script>

Everything is working except the response. I cant find anywhere what to expect as a response. Or if Im binding the data of elms correctly. Can anyone help me in this area?

The login required a csrfToken to be passed along with

 headers: {
                'Content-Type': 'application/json',
                'X-CSRF-Token': csrfToken
              },

and "?_format=json" appended to the url but for the registration values I am running blindly. Does anyone have any information on what to pass? The response I get is the html page.

Lastly, the Registration has a captcha v2 which outputs the error: "The answer you entered for the CAPTCHA was not correct"

I also looked at this page: https://www.drupal.org/node/2405657

Showing the #Register

POST: https://example.com/user/register?_format=json
Content-type: application/json

{
  "name": { "value": "fooBar" },
  "mail": { "value": "[email protected]" },
  "pass": { "value": "secretSauce" }
}
200 OK

I dont get a json response, it returns the syntax error: unexpected token < in JSON at position 1.

fetch(url, {
            method: 'POST',
            data:  JSON.stringify({
  "name": { "value": "fooBar" },
  "mail": { "value": "[email protected]" },
  "pass": { "value": "secretSauce" }
}),


            dataType: "json",
            contentType: "application/json; charset=utf-8",
            success: function(data) {
                console.log(data);
                successmessage = 'Data was succesfully captured';
                $("#overlay_container").text(successmessage);
            },
            error: function(data) {
                console.log(data);
                successmessage = 'Error';
                $("#overlay_container").text(successmessage);
            }
        })
        .then(response=>response.json())
        .then(json=> {
            console.log(json);
        }).catch(function(err) {
                        console.log(err);      // shows the above exception
                 });

None of the success: or error is responding, just the catch

Further more, when the _format=json to the url, the following error is reported:

{message: 'Not acceptable format: json'}
message: "Not acceptable format: json"
[[Prototype]]: Object

Next, I have tried to follow this posts response by @Fons Vandamme of installing Rest UI and configuring the user registration

user registration with rest

message: "No user account data for registration received."
[[Prototype]]: Object

Next: under admin extend, changed the authentication from cookies to basic authentication by installing the HTTP basic authentication, navigating to the rest UI configuration for the User registration /user/register: POST and the new error is a 400 Bad Request.