Use the build-in model 'f14' as an example, if you drag in a Knob block and connect it to the Gain block Zw, the changing of the knob position actually changes the value of the variable 'Zw' in the base workspace. You can move the knob, then check the value of Zw in the base workspace to see it changes. You could also change the value of Zw in the base workspace first, then update the model and see the knob position changes.
So the command you need to run is for example as below. You don't change anything regarding the Knob block. Rather, you need to change the variable for which it is connected to.
assignin('base','Zw',55)