You can model a Port as a relationship from an Association to a Classifier using the Association as Port symbol (indicated as UML2-5-1 Port under the Association Node to Class as Property line symbol on a UML Block diagram).
The definition of the UML Port line symbol (essentially Association Node to Class as Port line) is UML2-5-1 Port – the same Model Element that can be attached to a Classifier as a small rectangular Port symbol.
