Expressing a distribution in the model block

I am having trouble expressing a distribution inside the model block. Hence I am not able to upload the mod file. But this is what I want do.
I have an equation
Q(t) = pi(t) *p_h + (1-pi(t))*p_l

Where pi(t) is probability with which model takes the value p_h.
Pi(t) ~F(y(t) - y_bar)
Where F is a CDF with argument y(t) - y_bar where y is output and y_bar is steady state output. How do i express F in the model block? Can someone help please.
Thank you.

You probably have to use an external_function. Search the forum.