Initialise Simulink variables for one subsystem from a mask of another subsystem

I would like to have a 'Control panel' which has no connections. At the moment. I have created an empty subsystem and then created a mask for this subsystem, and added a Popup box with 3 variables, lets say, ['a' 'b' 'c']. Outside of this subsystem, ie, on the same level, I would like to be able to obtain the chosen variable and use as an input, ie a constant.

At the moment, I have named X my parameter in the mask. Then in the Initialization I have:

global const; const = X;

I then have a constant, where the value is set to const, which is connected to a display, outside of this subsystem.

However this does not work. Does anyone have any suggestions to how I can make this work?

This is a very simplified model, please don't suggest that I should use go to tags or data stores or something and replace this with the constant. This will not work.

Thanks in advance for any suggestions.