Controlling for pre-treatment trends in single-group ITSA study

[Scott Rick]

Hi. I am running the single-group ITSA specification below, which accounts for two interventions - a mandate and an update. B₀ is the intercept, B₁ the pre-intervention trend, and following which is the level change and the trend change for each of the interventions.

Outcome_it = B₀ + B₁T_it + Mandate_it + Mandate_itT_it + Update_it + Update_itT_it + e_t

I've been asked to include an interaction term that controls for pre-intervention trends, and I'm not sure how to do this. Any help on this front would be much appreciated.