I would like a Dome on Cargo ship how can i do that?

The problem is that Cargo uses a box-shapred zone rather than a sphere one. You'll have to ask Facepunch to add the ability to attach visible boxes to things.

If you just need it for admin debugging, try /dynpvp show in chat to see a temporary wireframe box (although it doesn't move with the ship).

Merged post

Update: This is in-work and should be included in the next release.

Feature is now released in version 4.4.0.