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Click Save, enable Preview and Debug mode and check if the tag ONLY fires when it is supposed to.
3- Creating a custom event in Adloop
Once the event is created, it is visible in Google Analytics under "Behavior" → "Events."
When connecting the Analytics data source to Adloop, you will be able to add one or more events manually:
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In this case it will be Event Name "404" and Event Category "404 Response".
You have to copy/paste what is written in Analytics:
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To then find the event in Adloop, it is very simple, just go to a performance report:
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In the metrics menu, you can go to Metrics by Source → Google Analytics and you can see your 404 appear!