Extract a Substring from Every Element in a List

Okay - I am following now.  This is the transformation needed to each item selected in the Slicer:

let
    ParameterText = "Mon, 01/09/2023 (Proposal 1)",
    ExtractDateText = Text.BetweenDelimiters(ParameterText, " ", " "),
    ExtractDate = Date.From( ExtractDateText ),
    DateToText = "'" & Date.ToText( ExtractDate, "YYYY-MM-DD" ) & "'"
in
    DateToText

Your code can be updated to this:

let
ParameterText = { "Mon, 01/09/2023 (Proposal 1)", "Mon, 01/10/2023 (Proposal 1)" },
//ParameterText = "Mon, 01/09/2023 (Proposal 1)",
selectedDates = 
    if Type.Is(Value.Type(ParameterText), List.Type) then
        "in (" &
        Text.Combine(
            List.Distinct(
                List.Transform( 
                    ParameterText,  
                    each "'" & Date.ToText( Date.From( Text.BetweenDelimiters( _ , " ", " ") ), "YYYY-MM-DD" ) & "'"
                    )
            ),
            ", "
        )
        & ")"
    else
        "'" & Date.ToText( Date.From( Text.BetweenDelimiters(ParameterText, " ", " ") ), "YYYY-MM-DD" ) & "'"
in
    selectedDates

Note the results:

With list is:   "in ('2023-09-01', '2023-10-01')"
With Single is: "= '2023-09-01'"

I am assuming you need to include the text with an Native SQL String.