A system (10) includes a support structure (20) associable with a bottom wall (BW) of a washing tub (WT) of a washing machine (WM). The system further includes a container body (22) having a receptacle (14) configured for receiving a quantity of washing agent. The container body (22) has an opening (24) leading into the receptacle (14). The container body (22) is mountable on the support structure (20) in the washing tub (WT). The container body (22) is configured for being reversibly brought by a user into a retracted condition and an extracted condition. In the retracted condition, the container body (22) is inserted in the support structure (20). In the extracted condition, the container body (22) is removed and separate from the support structure (20). The system (10) includes a closure element (16) configured for closing the opening (24).