As the question has been left unanswered, for future readers of the questions:
The documentation you link gives the answer to your question.
Given the fact you have a pre-trained model as you say:
YPred = classify(netTransfer,testDigitData);
netTansfer is the pre-trained model and
testDigitData is your test image that you want to predict the label.
Your image(s) need to be loaded following the instruction also given in the documentation:
digitDatasetPath = fullfile(matlabroot,'toolbox','nnet','nndemos', ...
digitData = imageDatastore(digitDatasetPath, ...
digitData will become
testDigitData if the images that you load are for testing purpose only, which seems to be your case)