I did it with a "Matlab function" which you can add to the simulink diagram like any other element.
When you double click the "Matlab function" the matlab editor opens, and you can use the following:
function y = fcn(u)
y = exp(-2*u);
Next I tied a "Ramp source" to the "Matlab function" input, and it works nicely.
If this is what you want please mark this as the answer.
