The split audio in djay uses one channel of your phone/ipad stereo out for headphone cue and the other to be mono out to the amp. You need a cable with 1/4" stereo on one side and 2x1/4" jacks on the other end - there are fancier, with different outputs, but this will work.

https://www.amazon.com/gp/product/B00E411NG6?ie=UTF8&th=1&linkCode=sl1&tag=djgear2k-20&linkId=77998f74374f6e14ecd29972948be038&language=en_US&ref_=as_li_ss_tl

You want to find pre-cueing, split output and turn that on.

The most inexpensive option would be to buy an audio splitter cable like this that you can connect to your laptop's headphone jack.

If you can, consider returning the Mixtrack and getting another DJ controller with a built-in sound card as a current (i.e., non-discontinued and works with current setups) audio interface that's made for DJ'ing can cost more.

To learn more about DJ audio interfaces in general, watch this video. It explains them very well.