I’m not sure how it can work, since each user would have to give you their credentials to sync through Make, and if I recall right, Make doesn’t let you set a dynamic “credential” as their input to run a module.
I think an alternative is to do the opposite, let them add tasks through the app, and allow them to add the event to their calendar through Agical. Obviously it would have been better if it works like you said, but credentials-wise, it’s a bit of a problem.