The process to add messages to Junk is to right-click on the message and choose Move to Junk. Or Menu > Message > Move to Junk.
If you choose the first option, the message will be moved to Junk but next time you get one from the same sender, it will remain in the Inbox.
The second and third options will move the message to Junk, and remember the address/domain so that next time they will automatically be moved to Junk.