When you see an event online, click on the Share Menu icon. This shortcut will appear at the bottom of the list.
If it's a Tweet, click the share menu icon and then click "Share tweet via" first to get to the list.
Then click "Online event cal entry".
The shortcut will ask what time the event is.
If you have any other appointments that day, it will display them so you can avoid setting up a conflict.
Then it will create the calendar entry with a 5 minute alarm before the start time. The calendar event will contain the original link so you can get back to it easily.