Manual:$wgAjaxWatch/en

Details
Enable watching/unwatching pages using Ajax. Requires $wgUseAjax and $wgEnableAPI to be true too. Causes wfAjaxWatch to be added to $wgAjaxExportList.

Other
If you want to add a Watch/Unwatch tab (the tab with the star), set $wgVectorUseIconWatch to true.