Hi David,

Thank you for contacting WPClever Support Forum. 

We’ve received your request and assigned one of our developers to help you with that. 

He’ll be looking into your question and responding with specific instructions as soon as possible. 

Please stay patient as we're having very heavy workloads, it might take some time for our developers to reach back.

Best regards.

Hi David,

Please add the below custom code (How to add custom code?)

/* WPClever */
add_action( 'wp_footer', 'woosg_check_all_btn', 99 );
function woosg_check_all_btn() {
    ?> 
    <script type="text/javascript">
      jQuery(document).ready(function($) {
        if (!$('.woosg-wrap').length || !$('.woosg-checkbox').length) {
          return false;
        }
        $('.woosg-products').
            append(
                '<div class="woosg-product"><div class="woosg-choose"><input type="checkbox" id="woosg-checkall"/></div> <label for="woosg-checkall" style="cursor: pointer">Check all products</label></div>');
        $(document).on('click touch', '#woosg-checkall', function() {
          $('.woosg-checkbox').prop('checked', this.checked).trigger('change');
        });
      });
    </script>
    <?php
}

And the result https://www.screencast.com/t/Jyv1D1K0D

You can change the text "Check all products" in the above code as you want.