Output function to obtain accuracy

I used the training option to plot the training progress. I followed the steps from here:

https://uk.mathworks.com/help/nnet/ref/trainingoptions.html?requestedDomain=true

Now I want to obtain at each iteration the accuracy and export the value in a text file. It seems the Outputfcn will do what I need but I do not understand how to get it run. Any help would be appreciated.