First, let’s create the DetrendedRevenue measure:

DetrendedRevenue = 
[Revenue] - [Sales Forecast - Trend]

Now we can create the Seasonality measure:

Sales Forecast - Seasonality = 
VAR CurrentYear = MAX('Date'[Year])
VAR CurrentWeek = MAX('Date'[Week])
VAR PriorYears = 
	FILTER(
		SUMMARIZE(
			FILTER(
				ALL('Date'),
				'Date'[Year] <= CurrentYear 
				&& 'Date'[Week] = CurrentWeek
			),
			'Date'[Year],
			'Date'[Week],
			"_Sales", [DetrendedRevenue], "_originalSales", [Revenue]
		),
		[_originalSales]<> BLANK())
RETURN 
    AVERAGEX(PriorYears, [_Sales])