Use the AeraOnline link below to nominate for a ride.
If you have never used it before you will have to register. Follow the prompts to create your login. Then set up yourself as a rider and allocate your horse in the system. Once done, go to the event you wish to enter, click on "View or nominate now" then click on the distance you wish to enter.
When the ride is available on AeraOnline for entries, you will see the corresponding link here.