Since you didn't install through community applications those fields weren't auto-filled for you. I am not exactly sure about the Icon URL field but I believe that just gives the docker an icon when looking at the docker tab instead of a blank image. However the WebUI field if how you would normally access the docker. It would be something like tower:portnumber or localip:portnumber but since it is plex we are dealing with, it doesn't entirely matter since you can access the actual plex server application from any client application with the admins login.
So in this case, you would be just fine to leave them blank. But if you want to fill them in, for the IconURL, just putting in a direct link to an image for the docker should work (not exactly sure about this one) and for the WebUI put the servers local ip (so something like 192.168.1.xxx with the port number 32400 since that is the default port plex uses.