Where should this seal ring from a syphon go?
I'm not sure due to different terminology but I think you're referring what we call where I live a p-trap. If so, the washer is most likely the slip-joint washer as seen in this photo:



There is one where the tailpiece connects the trap as well as one where the elbow connects to the trap's outlet. This second joint will always have to hold water as the trap remains filled past this point.

If this is not what you are referring to, a picture would help.