M query Filtered = #"Filtered Rows"{0}[Content]

Returns the first field value (Power query has 0 based index) after filtering.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUcrPS1WK1YGwS8rzEeyMolSIjBGQl5ZfWgTmGIM4mWWpCkqxsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}}),
    #"Filtered Rows" = Table.SelectRows(#"Changed Type", each ([Column1] = 1)),
    Custom1 = #"Filtered Rows"{0}[Column2]
in
    Custom1