How to integrate Spotify in my Wurlitzer jukebox application on Windows?

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Plan

Free/Premium

Country

Netherlands

Device

Desktop 
Operating System

Windows 7, 8, 10

My Question or Issue

I want to use the Spotify API to load music into my Jukebox that runs on Windows. What is the best way to authenticate the user to be able to get his own albums/playlists and artists in the jukebox? And without having to login each time he connects to Spotify with his or her account?
I made a new app in my dashboard and have a client and secret id now. What is the next step?
To get an idea, just take a look on my web page to see how I have made the jukebox. It's only capable at the moment to listen to offline music (Mp3) or videos (Mp4). I want to make it possible to use Spotify to integrate this in my jukebox. It would make it much more fun to use the jukebox this way I think.

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This is my website https://wurlitzermediaplayer.com/

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Anyone who can answer my question or share some desktop ideas for Spotify?