Link back from agenda to mail/source missing.

When creating an event using the quick link apple provided in the 'Mail' app, the event used to have a link back to the source of whatever created the automated event (in this case, the mail app). Recently this stopped working for some reason, and now the 'URL' part of newly created events is missing.

How does one turn this back on/get this option back?

Pictures below as an explanation.

^ Creating an event using the shortcut provided.

^ Old link back to the source of the created event. In this case: An email.