Custom Column filled with a specific value from another column

Hello - this is a way you can do it. 

Script

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WUtJRUorVQdCm+kZm+kYGRkYwAef8nPwiICc4syoVLJCUU5oK5PuCOUWpKXB2eUZmCUjGRyk2FgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    TargetColumnName = "Column1",
    TargetRow = 2, // row indices begin at 0
    // Alternative way to reference the target cell...
    // TargetValue = Source[Column1]{2} 
    TargetValue = Table.Column(Source, TargetColumnName){TargetRow},
    RemoveTopNRows = Table.Skip(Source,TargetRow+1),
    NewColumn = Table.AddColumn(RemoveTopNRows, "New Column", each TargetValue)
in
    NewColumn

Before

After