First of all, can you please share the URL of your existing application? Also, need to know what kind of issue you're facing on login side? Apart from that, what kind of loading screen do you want on your app? It will be better to connect and arrange a quick session regarding the same so the understanding will be the same on both sides and able to start this ASAP.
To get started, I need access to the code of the same so I will able to make the changes on the same.
Waiting for your response.
Thanks