Update a blank row if two rows in another column match

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.  The key is to replace your blanks with nulls to get the fill down to work.  Your sample data had a space, but adapt the Replaced Value step for your actual data.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQwMABiJR0lRxB2cjY0MgYyFJRidaKVfJ0dgbJArhOynKF5QIiHUmwsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Item code" = _t, AB = _t, Reference = _t, JobCode = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Item code", type text}, {"AB", type text}, {"Reference", type text}, {"JobCode", type text}}),
    #"Replaced Value" = Table.ReplaceValue(#"Changed Type"," ",null,Replacer.ReplaceValue,{"JobCode"}),
    #"Sorted Rows" = Table.Sort(#"Replaced Value",{{"JobCode", Order.Descending}}),
    #"Duplicated Column" = Table.DuplicateColumn(#"Sorted Rows", "JobCode", "Fixed Code"),
    #"Filled Down" = Table.FillDown(#"Duplicated Column",{"Fixed Code"})
in
    #"Filled Down"

Pat