How to redirect a node to an external URL?
Since you want this done in a view here is a possible solution


Add a link field to your content type external url which would need to be followed when available otherwise (if empty) it would take to node view (normal behaviour).


Modify the Fullcalendar View and add follow 2 fields 


Link to content hidden

external url and adjust the No results behaviour to include {{ view_node }} 

Use external url as the link 


NOTE: it appears that Fullcallendar view module takes the link to node in a way that it cannot be modified within the view as described above.  There is a feature request that asks for such an option.  In the meantime perhaps the above can be achieved with another calendar module () or modify the URL from within a custom module.