Measure value for first item only

Please define "First Row" - there is no built-in concept for that in Power Query. Do you mean sorted by DN#?

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("fc1BCoMwEIXhu8xa6GQmMyZLabEFEzALLSK5/zWqllio4u7Bx8+bZzDs2DBqTVBB7DBxCLdm2YKQq6MPlx6n6z5O7z9PzUPIe63t6s8+FLfuxGPbnbj1anjzPmFxQ7sLMimunnTg8b45/3qx6IS+/6/Sk0DOHw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Order No" = _t, #"DN#" = _t, #"Order Total Weight" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Order Total Weight", Int64.Type}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type",{{"Order Total Weight", "Total Weight"}}),
    #"Added Index" = Table.AddIndexColumn(#"Renamed Columns", "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Order Total weight", each if [Index]>0 and #"Added Index"[Order No]{[Index]-1}=[Order No] then null else [Total Weight]),
    #"Removed Other Columns" = Table.SelectColumns(#"Added Custom",{"Order No", "DN#", "Total Weight", "Order Total weight"})
in
    #"Removed Other Columns"

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done".