Find a trigger for first addition of a subscriber to a frontend user

Hello
We are writing our own plugin and need a trigger for the above subject.
We want to trigger our action only after the first addition to a list managed by a front end user. We were thinking at confirm but it does not work if the user is already in the DB and simply added to a new front end user list.

Mind to give a hint or do you have one somewhere ?

Thanks
Regards