how to set first value of a user created trend variable in EMS

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I have created a user defined variable in EMS and want to manipulate it. How can I define its initial condition i.e. set the first value of the variable to be used by later timesteps.

My equation is: set Ts = aa@TrendValue Temp_s 1 + bb(@TrendValue Temp_a_in 1 +cc)

here Ts is the variable I created and am trying to calculate and Temp_s is the trend variable associated with it. The initial condition I want to specify is: Ts (1) = Tain (1) + deltaTss/effectiveness

aa, bb, cc and dd have been calculated using a subroutine.

I'd appreciate any help.