The
external_user_id
is an identifier you set which helps map your unique database User ID's to the OneSignal
player_id
device record. This could be any unique identifier like Firebase User ID, CRM User ID, or even email if you choose.
However, I cannot find the setting to use the external_user_id in OneSignal settings
Hi
Are you able to share some more information about your Zap so that folks have some more context? What trigger/action are you using and what are you trying to do?
If you’re able to share a screenshot of where you’re seeing the external_user_id in the Zap that would be super helpful (don’t forget to remove/hide and private information from your screenshot). 
To add to
Just following up here to add that the external_user_id is set up within the Settings → Keys & IDs section in OneSignal. There is a help guide on the OneSignal website that goes into more detail on how to do that in more detail here: Identity Verification
Hope this information is helpful! :)
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