Delimiter

Ok. You've put the code I provided into a custom column, rather than as a new query with Advanced Editor. The code I provided was just an example of how to perform the steps on your query.

To apply this to your query, you would use this code in a new custom column:

Text.Combine(
    Splitter.SplitTextByPositions({0, 2, 4})
    ( Text.PadStart(Text.From([Code]), 6, "0") ),
    "-"
)

Full example query to paste into Advanced Editor:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQ0NTZQitUBsowMDM1gLANTKMvUzNgExrS0MAczLYHAEKgpFgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Code = _t]),
    chgTypes = Table.TransformColumnTypes(Source,{{"Code", Int64.Type}}),
    addCodeText =
    Table.AddColumn(
        chgTypes,
        "codeText",
        each Text.Combine(
            Splitter.SplitTextByPositions({0, 2, 4})
            ( Text.PadStart(Text.From([Code]), 6, "0") ),
            "-"
        )
    )
in
    addCodeText

Output:

Pete