Add the meta tag to the <head> tag in P4 Core: <meta property="fb:pages" content="Facebook_Page_ID" />
Add a field in the P4 Admin Settings: Facebook Page ID
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Facebook instant articles cannot be enabled via Google Tag manager (GTM), therefore they should be added somewhere below the <head> of the P4 sites. imohamme is also recommending this, because GTM does not support this (the Facebook crawler responsible for checking the meta tags cannot read tags in a GTM script).