Constituent Forms can be used to sign up for more information about a specific event. But that's only helpful if that can be linked to the Constituent so that you can filter to those who signed up. Please have Constituent Forms set an Action or Custom Field so the List can use that criterion for emails.
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A workaround that may be feasible short-term for this would be to add a custom field specific to the event (for example, use a Yes/No Custom field to "Sign up for more info on X event". This would be an additional click for the person filling out the form, but would act like an opt-in that you could then query off of.
We are exploring the concept of "hidden custom fields" which would be a custom field that you would control and could add to anybody who submitted through a form. I don't have a timeline for this yet, but hoping late 2026/early 2027.