To create the fraud0 Audience in Google Analytics, which can later be used in Google Ads, you need to trigger the custom event with the Google Analytics Tag first and check if the event is properly sent to your property.
Go to your Google Analytics Account
Navigate to your property and go to Events in the Real Time section in the left-hand navigation menu
If you get plenty of events into your account, it might be helpful to use the search field on the right side of the report and search for the value you’ve set for the event category (here: “F0flag”).
Now you should be able to see the F0-event.
The real-time report only shows data, that has been collected in the past 30 minutes.