Linking "Name field" in the front end form with the "Author field" in the post

Dear All,
As we know that a user has to login the backend to write a post. Once published, the post will have the author name as the user name (who posted the post). I made a front end posting through custom fields using a simple custom made form so that anyone can post from front end without the need to login.

However, I have a problem that the published post has no author name attached to it. So this is very difficult to name the person who posted it.

So what I want that how can I relate (or link) the “posting user name” field in the front end form with the “author or user ID” in the post. Or is it possible that my default all the front end posted posts will bear one name e.g. “Net User”.

Cheers,
Zan