How to create a custom loss function and to combine two losses using Keras

Question

I am working on a Neural Network with Keras and I want to add another custom function to the Loss term inside the model.compile() to regularize and somehow penalize it, which is the form:

model.compile(loss_1='mean_squared_error', optimizer=Adam(lr=learning_rate), metrics=['mae'])

I would like to add another loss function as a sum of the predicted values from the Loss_1 outputs so that I can tell the Neural Network to minimize the sum of the predicted values from the Loss_1 model. How can I do that (loss_2)?