@Rebecca-MSFT here is how I did it



"itemUrl": "https://tatatiago.sharepoint.com/sites/Test/Shared%20Documents/Forms/AllItems.aspx#id=%2Fsites%2FTest%2FShared%20Documents%2FABC%2Epdf&parent=%2Fsites%2FTest%2FShared%20Documents",
Split by sitename /Test/ = split(triggerBody()?['entity']?['itemUrl'],'/Test/')
Get tenant url = outputs('Compose_-_Split')[0]
Hope this helps.
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