If you prefer streaming over owning the MP3 file, "Just Sing" is available on all major streaming services: TROLLS WORLD TOUR | "Just Sing" Full Song [Official Clip]
For users in specific regions like India, you can download the MP3 directly through the Gaana App or JioSaavn with a premium subscription. Streaming Platforms troll world tour just sing mp3 download
If you are looking to add this track to your personal collection, here is everything you need to know about the song and where to legally find a . Where to Legally Download "Just Sing" MP3 If you prefer streaming over owning the MP3