How to determine the loss function of a LSTM network and change it?

I am trying to use the LSTM network for forecasting a time-series. Based on this documentation:

https://nl.mathworks.com/help/deeplearning/examples/time-series-forecasting-using-deep-learning.html;jsessionid=df8d0cec8bd85550897da63bb445

I managed to make it run on my data, I am just curious on what the loss-function is. How/where can I view this and how can I edit it, for if I would like it to minimize some other function?