At this time, Custom Events cannot be used in JQL. In order to capture the data from a Custom Event in JQL, you will need to recreate those Custom Events.
Why can’t I use Custom Events in JQL?
Mixpanel allows Custom Events to be created with both People and Event properties. Due to this flexibility, it makes it difficult to separate the Properties when it comes to running JQL queries for Events, People, or joined Events and People.
However, there is an API that you can use to quickly see your Custom Events and how they have been created. You will just need to replace with the API secret specific to your project:
The response will contain an "alternatives" key-value pair. This value should be passed into a JQL query as an "event_selectors" parameter.