Hm, i would try to make a replicated TextArray. If the player presses “Send”, the Client tells the Server to add this Text to the array and replicate it to all other clients. Now every client should have the same Text inside that array.
You can than use the array to display the text inside the chat with a for loop.
I guess you would need to make a function that is fired on every client everytime the array is updated. This function should either rebuild the UMG widget and in its contruct function you would add a text for each element inside the textarray. Or you could just update a widget by using a bool that is changed to true if a new string was added and than starts a for loop at its end the bool will be set to false again.
EDIT: I think with this you can also let the server check your player name for being an admin and filter admin comments like /give item 10 or something like that 