API:Watch

Pages can be watched (added to your watchlist) and unwatched (removed from your watchlist) with action=watch. Of course, you can only add and remove pages to/from your own watchlist.

Token
To (un)watch a page, a token is required. This token is the same for all pages (and is not different for watch vs unwatch), but changes at every login. Watch/unwatch tokens can be obtained via action=tokens with type=watch (MW 1.20+), or by using the following method:

Examples
Requests Must be "POST"ed, but for example's sake, "GET"s are used here:

Possible errors
Only the usual stuff.