With Twitch, you sadly can’t pick any track and use that within your broadcast.
The music you use in your stream must be licensed correctly or in the public domain.
With Synchedin, you are able to download any track in the Synchedin library and use this within your stream either as background or foreground music.
If you’re looking for more information on sync licensing, you can find out what is a sync license here and why you need a sync license here.
By subscribing to Synchedin for a small monthly or annual fee ($4.99 per month/ $49.99 per year) you will automatically obtain the sync license to use the music you’ve downloaded in any visual content you create, whether that’s Twitch, YouTube and even game design and filmmaking!