Using simulink "to workspace" variable as variable in a user-defined function

Hello,

In my Simulink model, I have several "to workspace" block as well as user-defined functions. For one of the function, let's say exp(a*u)+exp(a*u^2), I would like to use one of the "to workspace" values as 'a' (that is, not as input u, and changing while the simulation is running). Is that possible? If so, how?

Greetings, Kathrin