Assuming that Event Session is a single string/text field added as a checkbox field on the form to capture user's preferences for webinars, you could reference the data in the Event Session field to check if it contains any of the stored values corresponding to each webinar choice added on the form or not in the respective send email flow step's choice (just to be clear, there would be in total 4 send email flow steps, one for sending each webinar confirmation email given that they satisfy the condition in choice step). This should ensure that you send only the corresponding webinar confirmation email(s) that a user has chosen while filling out the form.
Alternatively, you could also create a single email and add the webinar details dynamically based on the user's selection of webinar(s) using velocity script. IMO from the UX-wise, I think it'd be better to send out a single email with all the details of user's choice of webinar(s), instead of sending out up to 4 emails.
Lastly, since this is a webinar confirmation email send campaign, you should set it as triggered campaign listening for the "Fills out Form" activity for the corresponding form.
Edit - If the webinar selection field is not added as a required field on the form, then if a person fills out the form again without making any selections, they would end up receiving emails per their last selections as submitting empty values in a field does not wash the existing value in it (in that the case, you’d want to update the field to NULL after sending out confirmation email(s) in the campaign so they don’t receive emails as per the old data in the field in case they resubmit the form w/o selecting any webinar(s)).
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