When adding your event to the website you will be guided through a few pages where you will need to enter information about the event such as name, dates, description and opening times. 

On the third page of this process, under 'Images and extra information', you have an opportunity to relate other events and organisations to the event you are adding.

Simply scroll down to 'Other Events' and type in the name of the event that you would like to relate to. You will also be able to select how the events are related.