SQL Server MERGE without a source table

Try this format:

MERGE TARGET_TABLE AS I
USING (VALUES ('VALUE1','VALUE2')) as s(COL1,COL2)
ON I.COL1 = s.COL1
WHEN MATCHED THEN

You could also reference this: "Merge" style operation with literal values?

You could do it like this:

Declare @customerID int = 1;
Declare @productID int = 1;
Declare @purchaseDate date = '1900-01-01';

MERGE dbo.FactBuyingHabits AS Target
USING (SELECT CustomerID = @customerID, 
    ProductID = @productID, 
    PurchaseDate = @purchaseDate) AS Source
  ON (Target.ProductID = Source.ProductID AND Target.CustomerID = Source.CustomerID)
  WHEN MATCHED THEN
    UPDATE SET Target.LastPurchaseDate = Source.PurchaseDate
  WHEN NOT MATCHED BY TARGET THEN
    INSERT (CustomerID, ProductID, LastPurchaseDate)
    VALUES (Source.CustomerID, Source.ProductID, Source.PurchaseDate)
OUTPUT $action, Inserted.*, Deleted.*;