A simple way for COUNTIF function in power query

Second column with countif added. 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlLSUTIGYkMwjtWBiMBEjcEipkCWKVTUCEXEBIzBugwMQCZYWqKYZQFiggmEXoQYVHcsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [C1 = _t, C2 = _t, C1Desired = _t, C2Desired = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"C1", Int64.Type}, {"C2", Int64.Type}, {"C1Desired", Int64.Type}, {"C2Desired", Int64.Type}}),
    AddedIndex = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(AddedIndex, "SUMIF_1_argument", 
        each if [Index] = 0 
        then 
            {AddedIndex{[Index]}[C1], AddedIndex{[Index]}[C2]}
        else 
            {AddedIndex{[Index]-1}[C1], AddedIndex{[Index]-1}[C2],
            AddedIndex{[Index]}[C1], AddedIndex{[Index]}[C2]}),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "SUMIF_2_argumentC1", each #"Added Custom"{[Index]}[C1]),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "ResultC1", (FirstIterator)=> List.Count( List.Select( FirstIterator[SUMIF_1_argument], each _ = FirstIterator[SUMIF_2_argumentC1]))),
    //second column
    #"Added Custom3" = Table.AddColumn(#"Added Custom2", "SUMIF_2_argumentC2", each #"Added Custom"{[Index]}[C2]),
    #"Added Custom4" = Table.AddColumn(#"Added Custom3", "ResultC2", (FirstIterator)=> List.Count( List.Select( FirstIterator[SUMIF_1_argument], each _ = FirstIterator[SUMIF_2_argumentC2])))
in
    #"Added Custom4"