Power Query - Lookup

See this code subirch 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQwUNJRMgKTfvlKsTrRUA6EjEwtBosZYyozAXPMUZSZIolBlZlhCplj0WkB5pkgK7PEFAK6FlUsFgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Employee ID" = _t, #"Manager ID" = _t, #"Is Manager?" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Employee ID", Int64.Type}, {"Manager ID", Int64.Type}}),
    #"Added Is Manager" = 
        Table.AddColumn(
            #"Changed Type", "Is Manager", 
            each if List.Contains(#"Changed Type"[Manager ID], [Employee ID]) then "Yes" else "No")
in
    #"Added Is Manager"

It is a straight forward if/then/else construct with one function. The Key is this function: List.Contains(#"Changed Type"[Manager ID], [Employee ID])

It turns the Manager ID into a list, then scans that list to see if the Employee ID is contained in it, returning True or False.

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.