Hold the Control key down on the key board (Ctrl) then left click the messages you want to mark as junk they should turn a light blue color.
Right click on one of the light blue junk messages and hover over the “mark” category and a sub menu will appear, press the option in the submenu “as junk”
Delete the messages from inbox. Right click again on one of the light blue selected junk messages again and chose the “delete selected” option.
All the messages should have been marked as junk and deleted, repeat from step one on any additional messages that you would like marked as junk in the inbox.