Third Party Widget in Checkout Page

Hi everyone,

can you please help? I have a widget from third party shipping service (maybe some of you know Packeta) that I want to place on my checkout page, where customers fill out shipping info, emails, etc. But so far I have not find a way to put it there. I tried code injection, custom CCS, advanced option in page menu, but nothing worked. I am attaching screenshot with the instructions and the code that I need to implement. 

Thank you in advance

Tomas

<script src="https://widget.packeta.com/v6/www/js/library.js"></script>

  <script>
    const packetaApiKey = 'd3e4bbb9eec4a4da';

    const packetaOptions = {
        country: "sk", 
    language: "sk", 
    valueFormat: "\"Packeta\",id,carrierId,carrierPickupPointId,name,city,street", 
    defaultPrice: "3.3"
    };

    function showSelectedPickupPoint(point) {
        // Add here an action on pickup point selection
        const saveElement = document.querySelector(".packeta-selector-value");
        saveElement.innerText = '';
        if (point) {
          console.log("Selected point", point);
          saveElement.innerText = "Address: " + point.formatedValue; 
        };
    }

  </script>

   <button class="packeta-selector-open" onclick="Packeta.Widget.pick(packetaApiKey, showSelectedPickupPoint, packetaOptions)">Select pick-up point</button>    
   <div class="packeta-selector-value"></div>

And isn´t there a way to access it through some developer options?