Field update on Event when related Potential is won

I am creating an automation workflow to update a (custom) field on an Event if the related Potential is set to "Closed Won".

When I create the Automation Workflow on Potentials, i can only update fields on Potentials, Accounts, Campaigns and Contacts. My field update would be on Event, actually the Event that is related to the Potential.

Can anyone help?

Thanks,