JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script src="https://raw.githubusercontent.com/1000hz/bootstrap-validator/master/dist/validator.js"></script>

<form accept-charset="UTF-8" method="post" role="form" id="bookform" data-toggle="validator" enctype="multipart/form-data">
  <div class="form-group col-md-6 col-xs-6 has-feedback" style="padding:5px">
  <input id="b2b_customer_name" name="b2b_customer_name" class="form-control input-lg headerform" type="text" required placeholder="Your Name *" pattern="^[_A-z\s]{3,}$" data-error="Please enter atleast 3 alphabets">
                        <div class="help-block with-errors"></div>
                     </div>
                    <div class="form-group col-md-6 col-xs-6 has-feedback" style="padding:5px">
                        <input id="b2b_cust_phone" name="b2b_cust_phone" type="tel" class="form-control input-lg headerform" type="tel" maxlength="10" placeholder="Mobile Number. *" pattern="^[789]\d{9}$" data-error="Please enter a valid mobile number" required>
                        <p class="help-block with-errors"></p>
                    </div>
                    <div class="form-group col-md-6 col-xs-6 has-feedback" style="padding:5px">
                        <select id="b2b_service_type" name="b2b_service_type" class="form-control input-lg headerform status" required>
                                        <option value="" disabled="" selected="" hidden=""> Service Type </option>
<option value="22">something</option>                                
<option value="34">Another</option>
                        </select>
                        <p class="help-block with-errors"></p>
                        </div>
                        <button id="registerbutton" type="submit" class="btn btn-xl submit" disabled> Submit </button>    
                  </div> <!-- Book div -->

JavaScript

$('#registerbutton').attr('disabled', 'disabled');

function updateFormEnabled() {
    if (verifyAdSettings()) {
        $('#registerbutton').attr('disabled', '');
    } else {
        $('#registerbutton').attr('disabled', 'disabled');
    }
}

function verifyAdSettings() {
    if ($('#b2b_service_type').val() != '') {
        return true;
    } else {
        return false
    }
}

$('#b2b_service_type').change(updateFormEnabled);