Power Query - Current Row & Row Above with conditions

Hello rsbin,
You are on right path, I'm extending a solution that will able to complete your requirement.

1. Create a Duplicate copy of Table 1 (original table) as Table 2
2. Then add index column to the table 1 starting from 0
3. Repeat the point 2 for table 2 starting from 1
4. Then merge table 2 with table 1 based on Index column
5. Expand the "Prev_GL_Prefix" column
6. Add new column using below M-Language

= Table.AddColumn(#"Expanded Table 2", "Custom", each if Text.StartsWith([Financial Row], "Total - ") and [Prev_GL_Prefix] <> null then [Financial Row] & " - " & Text.From([Prev_GL_Prefix]) 
else null)​

• Here are the working table snippet
  Table2
  

  
  Table1 M-Language

  

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("fY67CoNAEEV/ZbFW2JFYpM5DhAQL7cRiCJOwsM6adcz3Z11SaAip5nHP3Lldl+xAA2iVqXokj2IcT2EoUShJFzHp0wjt8wVq0NKkjma6uZllRbRO0G5cgsiztVH9vS1Aaw3hqGIhn7XkB8PBpfX4IhvI4mMeKsT3h7psvqNezF22bA4r9vSczTgQi7oi44Nim6mzJVqf/cnfvwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Financial Row" = _t, GL_Prefix = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Financial Row", type text}, {"GL_Prefix", Int64.Type}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    #"Merged Queries" = Table.NestedJoin(#"Added Index", {"Index"}, #"Table 2", {"Index"}, "Table 2", JoinKind.LeftOuter),
    #"Expanded Table 2" = Table.ExpandTableColumn(#"Merged Queries", "Table 2", {"Prev_GL_Prefix"}, {"Prev_GL_Prefix"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Table 2", "Custom", each if Text.StartsWith([Financial Row], "Total - ") and [Prev_GL_Prefix] <> null then [Financial Row] & " - " & Text.From([Prev_GL_Prefix]) 
else null)
in
    #"Added Custom"

Thanks,
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