Power Query (M) _ Dynamically update a column list for List.Sum function - excel

Question

I'm not sure if even possible but the goal is to dynamically update a query based on the user selecting a date. I have a table in my Excel file while updates a value which feeds to PeriodString variable (below)
/*Parameter name = PeriodString */
let
Source = Excel.CurrentWorkbook(){[Name="PeriodString"]}[Content],
StrPeriod = Source[Value]{0}
in
StrPeriod
The part of the code I want to update is the [ ..months selected ].
=List.Sum({[FYOpening],[January],[February],[March],[April],[May]})
With the below variable
=List.Sum({PeriodStr})
I tried using Table.Column as I realize I have to convert the value to a list of selectable columns but I cant' get it to work.
=List.Sum({Table.Column(PeriodString{0},PeriodString[0])})
Expression.Error: We cannot convert the value "[FY Opening],[Januar..." to type List.
Details:
Value=[FY Opening],[January],[February],[March],[April],[May]
Type=[Type]
Let me know if possible / alternatives.

Answer 1

If you need exactly value like "[Col1],[Col2],[Col3]" for PeriodString, then use such code:
let
Source = #table({"a".."e"},{{1..5}, {6..10}}),
PeriodString = "[b],[d],[e]",
sum = Table.AddColumn(Source, "sum", each List.Sum(Expression.Evaluate("{"&PeriodString&"}", [_=_])))
in
sum
I'd prefer to use PQ list instead:
let
Source = #table({"a".."e"},{{1..5}, {6..10}}),
list = {"b","d","e"},
sum = Table.AddColumn(Source, "sum", each List.Sum(Record.ToList(Record.SelectFields(_, list))))
in
sum