How do I make chat pop-up

Hello,

I'm very new to this,
how do I make the chat pop-up while I am in-game? I want Discord chats to pop-up every time someone is sending me a message and I can quickly hit a key (like overlay) and reply to them... not sure what am I missing here, when people send me chats I only hear the sound but can't see anything (in-game) and for some reason overlay won't activate... any ideas?

Thanks!