Referencing a value in the same row but in dynamic columns

You can use the Record.Field method:

let
    Source = Excel.CurrentWorkbook(){[Name="Table2"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Sales in EUR", type number}, {"USD", type number}, {"SEK", type number}, {"PLN", type number}, {"CAD", type number}, {"AUD", type number}, {"Currency Code", type text}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Sales in Currency Code", each  
        if [Currency Code] = "EUR" 
            then [Sales in EUR] 
            else Record.Field(_, [Currency Code]) * [Sales in EUR] , Currency.Type)
in
    #"Added Custom"