First create the 'event' by going to Rail Timetable - Edit Events - and scroll down to the 'new event' section

Enter the event title, a description, the link to the event and that for the tickets

Then tick 'use custom background colour'. I've chosen purple (217, 2 ,217) as the background with white text. Then save or update.

Then go to edit calendar, select the month and scroll to the date and add the event from the dropdown

Then test it and you're done!