Hello Chris,
The correct port is indeed message server's, which will be reached and queried about all the available instances on the system and with this information, web dispatcher knows which instance / system to forward the request to.
After setting accordingly, you can test with the following command:
sapwebdisp -checkconfig pf=/path/to/webdisp/profile
This command will retrieve the information from every system connected from the configured message servers.
Cheers,
Ian Segóbio.