Where is the Force Touch API for Apple Watch?
There isn't one (see Apple Staff answer here)
WatchOS 2 does sense pressure as a range, but there is no direct access to touch events in watchOS 2 - no API - so not progammable.
"Menus" are currently the only way for your app to respond to a force-touch (but the level of pressure required to trigger that event is determined by watchOS 2 - you have no control over it).