First of all, I hope you are having a wonderful day. Unlike @usr_updated, I know this is possible with the default Roblox chat system that needs to be tweaked a little. Let us get started. First, you need to copy the default chat system, so run the game.
Once you run the game, you should see the default Roblox chat. Copy all of them, stop the game and paste it back in. After pasting it in you need to go to the script below.
Open the script and find the function called methods:CreateBaseMessage and add the line highlighted below.
And that’s it. The results:
There is also a way to enable rich text in the chatbox but I am going to leave that to you to find out if you cannot you can always ask me I will help you.
Hope that works for you. Have a great day.