Offizieller Kommentar
NEVER connect it to a minijack on the computer as most cases that's a mic input, not a line input, and the line level from the mixer would totally distort the audio.
You're using a DJM-900nexus, why not use the built-in audio interface to stream to your software? Look up the OBS guides here; a new updated guide is coming later as I've just learned of a new ASIO plugin that can negate the need to use a couple of 3rd party apps.