Though for some potentially universal ones you could have something like:
-
I accept the terms and conditions
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I am not a robot
But a change to the trigger so you could have ‘user created/activated’ for this and not need a superfluous custom field would be 
Just a couple of extra notes now I’ve given it an actual run-through 
Probably obvious, but for topic_id you can also use a PM topic id (with the added benefit of a shorter email window delay, in case time was of the essence)
One potential downside is that the post is created when a user signs up rather than after they activate their account/verify their email. Though this may not be an important nuance.
You also want to make sure the post creator is the system user (or similar alternative) as you won’t get notified of your own posts if you use yours as the sender.
But just to confirm, a confirmation box custom field works just fine. 