How to filter a custom chat?

You can use TextService:FilterStringAsync() then use TextFilterResult:GetChatStringForUserAsync().

-- in the part of your script where message will be sent
local TextService = game:GetService("TextService")
local Result = TextService:FilterStringAsync(Message, Speaker.UserId) -- "Message" is the string of the speaker's message; "Speaker" is the Player instance of the player that wrote the message

local Players = game:GetService("Players")
for _, Player in ipairs(Players:GetPlayers()) do -- gets every player in the server
	if Player.Name ~= Speaker.Name then -- if Player is not Speaker
		local success, filteredMessage = pcall(Result.GetChatForUserAsync, Result, Player.UserId) -- Result:GetChatForUserAsync() but wrapped in pcall (protected call) to avoid errors that will break the script
		
		if success then -- no errors
			-- send the filtered message
		end
	end
end
-- the rest of the script

Make sure the script is a normal script or else it won't work. You have to get Speaker and Message somehow, probably by using a RemoteEvent. In sending the message, you should use RemoteEvent, too, to respect the receiver's chat filter settings.

As what @Ziffixture said, it is correct that you should search your problem before posting it here to avoid spam. I searched it and there's a ton of results with the same problem as yours and they are solved. Next time, search before posting, ok? :)