How do I set shutter mode to external?

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I have a flir lepton 2.5, I am using a purethermal breakout board, on a raspberry pi. I cannot for the life of me figure out how to change the mode of the shutter to external. I think I need to pass a lep_sys_ffc_shutter_mode_obj_t, But I don't know how to do this. I am trying to modify the existing raspberry pi qt program provided in the groupgets lepton module repository.