Very simple, you just need to type \n[x] where
x is the ID of your hero. If you are renaming the first hero in the database, you just need to type \n[1] in the message in Show Text.
Let's say you want him to say "Hey there, Mike!"
Type "Hey there, \n[1]!"
And when you play the game, the man will say "Hey there, Mike!", replacing the \n[1] with whatever name the player renamed Mike.
Also next time please use the in-built RMVX Help topics.
Have a nice day and Happy New Year in advance