How to sign up/log in in a custom plugin?

import { createAuthEndpoint } from "better-auth/api";
import type { BetterAuthPlugin } from "better-auth";
const myPlugin = () => {
    return {
        id: "my-plugin",
        endpoints: {
            getHelloWorld: createAuthEndpoint("/my-plugin/hello-world", {
                method: "GET",
            }, async(ctx) => {
                return ctx.json({
                    message: "Hello World"
                })
            })
        }
    } satisfies BetterAuthPlugin
}