You can pull up and start orders on
MDE to see what you can and cannot customize. Some options allow some customization and some do not. I just pull up and check to see. If I cannot change something, I either order in person or choose something else.
FYI: The restaurant or park has to be open when trying to see what options there are. The other day m, about 5:30 am, I wanted to check something on mobile ordering and I couldn’t as the place was closed.