" #Trigger it from yearly pulse, or perhaps the on_new_holder on_action to be more immediate. "
What does this bit mean? Where do I have to put on_new_holder?
Do I actually need is_triggered_only = yes if I just want the event to trigger with the dynasty ID and landed title?
Looks like
this page will be helpful too

Events have to be triggered by something; something has to make them happen.
One way is to have the game do it randomly, at some point in time. You will want to do this for the random events unrelated to specific and immediate affairs. E.g. "Introduce insane law to your realm". The way to do that is to specify a mean_time_to_happen block and in it say how often you want your event to trigger, generally speaking. It's in terms of "once every fifty years" or "once every two days".
For your case, you do not want that. You want an event that's triggered by something specific, something acute (the dynasty guy gaining a new title). That's what is_triggered_only = yes means; something is going to trigger this event, so do not bother to use mean_time_to_happen logic for it.
Once you've decided that
something is going to trigger your event, you need to decide what exactly. The simplest solution is to have the event triggered by a different event, but then you have to find something to trigger
that event. Another is to make use of an
on_action.
Read the wiki page for how to use it, but basically vanilla maintains a list of "happenings", and lets you specify events that happen upon such events occurring. One of them is "on_birth", which triggers every time someone new is born (and only triggers for the baby). "on_new_holder" is triggered whenever a title is held by a new individual, say if your title is held by someone of the specified dynasty.
By the way, I forgot something in my template. You will want to have the event set a global flag upon firing, and check for the existence of that flag in the trigger, to make sure it only happens once per gameplay. Use
this page for info about flags.