Oh weird. There's supposed to be a script for that, but there isn't. This project is a mess when it comes to making things clear for people, gee. I'll just show you what the script is SUPPOSED to do.
So to add an event right after the NPC finishes their dialogue, you place this chunk of code after creating the text box:
with (Event) {
ds_list_add(event_list, SCRIPT_NAME_GOES_HERE, ARGUMENT_GOES_HERE);
}
Once the NPC is finished talking, Event will run the next item in that list, which will be the event you just added.