Act as Proxy Manager - get person number

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Hello,

Is there a way to know person id or person number of the manager that I proxied as?

I would like to use it in a fast formula that would return a value based on a person used in Act as Proxy Manager - I'm able to get ID of a person who is logged into the system (value set with a query on logged user), but I would like to have it for proxy as well.

Thank you!

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