Administrating a customer connection with "a few black boxes"

[ramdyne]

I am trying to administrate a circuit that goes through my network and the network of a supplier. See the diagram below:

So the circuit goes from port Lan1 on switch A (where it is tagged as vlany) via a cable from wan1 to port px on switch B, somehow through a network that is a black box to me, (where I just have circuit id) into switch C where it exits port px as VLAN Z (tagged)

We have been able to create a circuit from wan1 on switch A to px on switch B, and bridge as vlany to port lan1. This doesn't feel right as a first time user, but it kind of does what I want to do. There might be other circuits here going through wan1, but there could be more wan ports with their own circuits, so front/rear ports don't work for us.

I haven't been able to think of a way to extend it towards port px on Switch C.

(The docker container my colleagues installed reports itself as "38c27a8947ba (v3.1.6)")

Does anyone have a hint?

Thanks!