Model with foreign exchange interventions

I need a dynare code for a simple model in which there are two equations for monetary policy, one for the interest rate and the other for foreign exchange interventions.

I wanted to implement a model by Benes in dynare, but unfortunately I don’t know how to declare to dynare that some variables are the trends of other variables and some are the steady state variables. when I declare them as new variables Dynare considers them new variables and they should be expressed in terms of the other variables for the model to be solved. I end up with more endogenous variables than equations.
here is the paper … -vavra.pdf

I want a dynare code of the model in this paper or please tell me how to deal with the steady and trend variables in the model.