Adding serial port triggers for entire block rather than individual trials


I’m a newbie and have used the builder to create an emotive stroop test. There are 4 experimental blocks for which I need to record fNIRS data for.

However, I can only find info on how to send triggers for individual stimuli/trials. What I am trying to do is send a trigger at the beginning of each block of 20 trials, and one at the end.

Any help would be much appreciated.