Insert row with reference to an existing row - Power Query...

Hi Team,

Looking for some help inserting a new row (record) to my 2 column table based on the minimum date in one of the columns.

Here's my table: 

And I want it to look as follows:

Where a new record has been inserted with 'Week_Index' = 0 and 'Start_Date' = min of all existing dates - 7 days.

I have the following code, but the syntax is not quite right:

Custom1 = Table.InsertRows(#"Renamed Columns",2, { [Week_Index = 0, Start_Date = List.Min([Start_Date]-7)] })

Can anyone help, please?

Thanks,

Matty