How to use an app(.mlapp) in simulink callback?

Hi, I have a simulink model I want to change the simulationTime and StepSize. But I want to use an app for this because my customer requires this. I tried using PreLoadfnc callback but the .mlapp did not even open. But if I use it in InitFcn it opens yet does not change the default values. This is my code line:

load defaultValues; % this line is used to load the default values
run('C:\Users\Desktop\ProjectA\SetParam.mlapp'); % this line should change the value of two parameters