Manual:$wgAjaxWatch

Details
Enable watching/unwatching pages using Ajax.

Requires useAjax>Special:MyLanguage/Manual:$wgUseAjax|$wgUseAjax to be true too.

Causes wfAjaxWatch to be added to ajaxExportList>Special:MyLanguage/Manual:$wgAjaxExportList|$wgAjaxExportList.

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