An attachment (1) for a liquid container with a container outlet, wherein the liquid container is surrounded by a container wall and is intended to receive a first liquid, wherein the attachment (1) has a bearing surface (2), which defines a bearing plane (A), for bearing in a sealing manner against the liquid container a reservoir (4) for a second liquid, a channel (6), which connects the container outlet to an outlet (10) of the attachment (1) and a passage (44), which connects the reservoir (4) to the channel (6), wherein the attachment (1) is equipped with a clamping lever (14), which extends in a use position through the bearing plane (A) and the container outlet and which has a pressing surface (32) facing the inner side of the liquid container in the use position.