This article is a Work in Progress. It may contain out of date or incomplete information.
As of version 0.62.0 the Harpoon Turret is currently not implemented in the normal game of Kenshi but can be accessed through the in-game editor (Shift + F12). It is located under the 'Defence' section of the building menu. After placing a Harpoon Turret there are two interaction options available: 'null' and 'dismantle'. Selecting the 'null' option will cause your characters to move forward and they will do the default labor animation. However, since turrets and ranged combat are a work in progress and not currently in the game they serve no purpose. Presumably, the 'null' option will be replaced by an action to use the turret in the future.