Do the content for the Everwebinar invite, registration and attended emails/streams remain the same across all four sectors? If so, you could probably just accomplish this by using one Event Program for your webinar and requesting the campaign. The reason being is that the emails for the webinar are not of a set cadence like you have in an Engagement Program stream. To do this, you would first add a Campaign is Triggered (Source --> Marketo Flow Action) as a trigger for the Smart Campaign in which you are sending the invitations. This would be in the Event Program. Once you activate the Trigger Campaign, you would go back into whatever Nested Default program you'd want to request this campaign. Since you want to do this based off of a specific activity taken, you can create a Nested Default Program within the Engagement stream that utilize the Flow Action of Campaign is Requested. This way, you can utilize that one Default Program across all four sectors. If you haven't already, you should import the Marketo BP Event Program Template for webinars, which will get you a good start on the campaign flow. Based on what you are describing, you'll need some modifications, but I think this could be a suitable solution for you.
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