It does not return an instance, it returns a TextFilterResult. Additionally, keep in mind:
This method currently throws if fromUserId is not online on the current server. We plan to support users who are offline or on a different server in the future.
You should not be using a hard-coded UserId, instead get the LocalPlayer’s UserId like so:
local Players = game:GetService("Players")
local TextService = game:GetService("TextService")
local client = Players.LocalPlayer
local filterResult = TextService:FilterStringAsync("MAGMA", client.UserId)
Keep in mind this function can error and it’s best to wrap it in pcall (protective call).