Distinct count in custom column

Here's one way to do it in the query editor.  To see how it works, just create a blank query, open the Advanced Editor and replace the text there with the M code below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclSK1YGRTmDSGYl0AZOuYNINRsYCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Customer = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Custom", each let thiscustomer = [Customer] in Table.RowCount(Table.SelectRows(Source, each [Customer] = thiscustomer)), Int64.Type)
in
    #"Added Custom"

Pat