Extension methods in Roblox-TS

I’m using Roblox-TS and wanted to take advantage of extension methods in Typescript, but RTS introduces it’s own string methods, and I can’t seem to add any.

Here’s what I tried:

interface String {
    startsWith(char: string): boolean;

String.prototype.startsWith = function(this: string, char: string): boolean {
    return this.sub(0, 1) === char;

This throws two errors from RTS - 'prototype' is not supported! and Using values of type 'any' is not supported! Use 'unknown' instead. Apparently RTS doesn’t support prototypes, but as far as I know, you need them to create extension methods. So is there another way or is it just unsupported