A method for producing a container from a preform having a body and a neck defining an inner cylindrical opening. The method including the steps of placing the preform in a mold having a molding cavity, —forming the container by injecting a forming fluid into the preform, and —filling the container with a liquid. Prior to forming of the container, an extension device is attached to the neck of the preform. The extension device includes an extension part defining an inner opening extending in the continuity of the inner opening of the neck. The extension device remaining attached to the neck of the preform at least during the forming and the filling steps.