"Je vais mettre un mot ici."
Traduction :I am going to put a note here.
The difference is a little subtle. It is correct to say "in here" when you enter a room or building, somewhere inside. It's cold in here. It's dark in here. It's pretty in here.
By just using the word "here", you can be talking about a general location, not usually inside but it could be inside. Wow! It's cold here. (in Alaska) Why are you here? (why are you present?) Why are you in here? (why aren't you outside?)
Normally when we are specifically talking about the interior location of something or someone, we say "in here".