Hi,
You can't do that in a Master page, because master page is due to the fact that the master page is not a "page" but a user control and is treated like one inside the Page. So no use having that property as it would only work for a actual pages.
But you can set it for one single page in the head secion, or all pages in the Web.Config file in the Pages Element.
Hope it helps.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support