In a function, use this.Sandbox.IO, see this post for some info, you don’t need a reference for it.
Sandbox.IO doesn’t replace all of System.IO, just the parts they don’t want you to use, you still can use a reference to System.IO and use the ‘unbanned’ bits of it.
(ps. this works since at least 2025.1, don’t remember if 2024.2 also)