This is a snap of non-logged in user (aka guest). But not for users or admins.
To trigger autocomplete in [[Special:Ask]] one needs to type "?" in to "Additional data to display" text box and pressing the down arrow. A popup will come up below the text box with all Properties defined on the wiki. This is the case, AFAICT, only if one is a guest (not logged in). If one is logged in it won't work.