Hello @ryanchang ,
Welcome to the forum!
The portainer is a GUI for docker (not only, but mostly). It is not replacement for ssh, so you cannot use it to run docker commands directly.
So you need either enable/install ssh server on your NAS and connect via ssh client from your PC to it then use the docker commands from our guide or try to use the portainer’s GUI.
If you would go with GUI, it’s listening on 9443 port by default, so you should try to use such URL: https://your-wd-nas-address-here:9443
Then you likely will be requested for login and password or perform a setup, if you didn’t before: Docker Standalone - Portainer Documentation
Next you need to learn how to run containers using this GUI, you should read their documentation, how to do it. Then you would need to convert CLI options for the container from our guide to their GUI analogues.
I would recommend to go with ssh though instead of using this GUI, but well.