How to get Facebook Pixel ID?
Mulwi Facebook Pixel uses Pixel ID and Conversion API Access Token to correctly track events (server-side), ensuring on one hand that Facebook accurately targets your ads and on the other hand, that you receive correct ROI values.
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Log into your Facebook Business Manager: https://business.facebook.com/
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In the left-hand menu options, click "All tools".

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In the opened menu, find "Events Manager" and open it.

Alternatively, you can open Events Manager directly via link: https://business.facebook.com/events_manager2/overview
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In the Events Manager, click on Data Sources.

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In the Data Sources, you can view all of your data sources and Pixel ID.

 
In case you haven't created a Facebook pixel yet, you should follow the Facebook manual: https://www.facebook.com/business/help/952192354843755?id=1205376682832142