How do i make chat apear in the chat box from npcs?

Whenever NPC chats (or when ChatService.Chatted is fired), you will make a system message by using StarterGui:SetCore("ChatMakeSystemMessage")

This LocalScript must be inside StarterGui.

local Players = game:GetService("Players")
local ChatService = game:GetService("Chat")
local StarterGui = game:GetService("StarterGui")

ChatService.Chatted:Connect(function(partOrCharacter, message, color)
    if partOrCharacter:IsA("Model") then
        if partOrCharacter:FindFirstChildOfClass("Humanoid") then
            local player = Players:GetPlayerFromCharacter(partOrCharacter)

            if player ~= nil then -- if the one who chatted is an NPC
                local configTable = {
                    Text = string.format("[%s]: %s", partOrCharacter.Name, message),
                }

                StarterGui:SetCore("ChatMakeSystemMessage", configTable) -- makes the message appear in chat box
            end
        else
            local configTable = {
                Text = string.format("[%s]: %s", partOrCharacter.Name, message),
            }

            StarterGui:SetCore("ChatMakeSystemMessage", configTable) -- makes the message appear in chat box
        end
    elseif partOrCharacter:IsA("BasePart")
        local character = partOrCharacter:FindFirstAncestorWhichIsA("Model")
        local humanoid = character:FindFirstChildWhichIsA("Humanoid")

        if character and humanoid then
            local configTable = {
                Text = string.format("[%s]: %s", character.Name, message),
            }

            StarterGui:SetCore("ChatMakeSystemMessage", configTable) -- makes the message appear in chat box
        else
            local configTable = {
                Text = string.format("[%s]: %s", partOrCharacter.Name, message),
            }

            StarterGui:SetCore("ChatMakeSystemMessage", configTable) -- makes the message appear in chat box
        end
    end
end)