Spoiler
The multi-select is a good idea if you plan to set the webinar attendance property manually, but if you’re setting this mostly by workflow as I suggested, I think you’re probably better off leaving it as a text field, since you’ll use each Webinar title once in bulk.
I say use the title once, because I’d probably use a naming convention like “[Webinar Title], [Date]” so i’d know which instance of a particular webinar the prospect attended.