Help with chat, adding a word to your sentence?

So I’m working on a sick GUI and I need to make it so when you press a button, I know how to do a mouse click detection, who doesn’t lol but my problem is how would I add the text to the little chat bar if its locked in coregui? Is that even possible?

Now I’ve been scripting for a very long time with the LUA Programming Language and have I never really used the ChatService, however this link should help you a little in understanding how to send messages into the ROBLOX Default Chatbox, OR are you trying to send messages using a custom made chatbox (Which is not really recommendable)

I was thinking of adding a word to the chat bar (where you type your message), here’s an example:
You click a button that says “Hello!” and In the chat bar you have the message “I’m spec” and it adds “Hello!” in the chat bar to make your message “I’m spec Hello!” and then you can send. I don’t know if this is possible and I know you could prob do this with a custom chat gui but I don’t really wanna use a custom chat gui.