Hi vishwajeet_hol , you'd need to build a custom component and use that in the message quilt.
From http://community.lithium.com/t5/Community-API/bd-p/developers-rest-api?leaf-id=User.profiles#User.profiles
The field you want is "name_first", but you'll need to make sure you've got a fallback for people who don't have their name entered and for people who have changed their settings to not show their identity.
This is a big change to make, is your userbase going to be aware in advance they will be posting using their own name and not a username?
And for UI consistency, are you planning on showing the name as their name "John" instead of "JohnD" everywhere?
If so, how do I @mention someone when I don't see their username anywhere?
How do I send someone a PM?
Usernames are unique, but first names are not, how do you address 3 John's posting in the same thread?
I think there's a lot to think about in implementing this. I'd love to know more on why you're heading down this route :)
Tim