Just @ and put the name of the one you want to mention without any spaces after like this: @Hitogoroshichan
Don't use this @ from Nippongo IME or keyboard.
- last page
The usual people do when replying at posts would be to simply type the target person's name then adding "@" before it, but if you want the link to go to a particular post that person made instead of that person's profile, you need to copy the link at the post's upper right corner (the one with the clock symbol) and use it as a URL tag for the name of the target. For example: