This value is not directly stored in the generalized linear model object, but it can be accessed by performing a deviance test. The following code will give the desired value.
d = devianceTest(mdl);
d.pValue
For more details on using 'devianceTest' for your model you can refer to the documentation link below https://www.mathworks.com/help/stats/compactgeneralizedlinearmodel.deviancetest.html
