In a simulink model you can specify callbacks. In this callbacks you can place your variable declaration.
In the menu "Flie" -> "Model Properties" -> "Model Properties" in the Tab Callbacks you can choose PreLoadFcn. There you can define your variable (eg. f = 1.234;).
An other possibility is to define the variable in the model workspace. This can be done in the "Model Explorer". (View -> Model Explorer -> Model Explorer)