The model initialization done top-down where the Model Initialization Commands (Simulation + Simulation parameters... + Initialization tab) are run first, then the various masked subsystem initialization commands (right-click component + Subsystem + Edit mask... + Initialization tab).
Your question suggests you want a bottom-up approach, which is to modify the Model Initialization Commands behavior from a dialog setting in a masked subsystem. That is not possible with PLECS.
What you can do is assign a variable in your Model Initialization Commands (e.g. "FET_bridge_device = 2") and then assign that value to your Combo Box by clicking combo box in the block parameters window, opening the drop-down, an then replace the "<reference>" string with the variable you defined ("FET_bridge_device").