Here is a sample code for generating a session.json file

[qianmianyao]

func main() {
	sock5, err := proxy.SOCKS5("tcp", "ip:port", &proxy.Auth{}, proxy.Direct)
	if err != nil {
		fmt.Println("proxy error")
		panic(err)
	}
	dc := sock5.(proxy.ContextDialer)
	ctx, cancel := context.WithTimeout(context.Background(), time.Second*20)
	defer cancel()

	apiID := 123456
	apiHash := "1d096xxxxxxxxxxx45f7c3"

	// save session file
	sessionStorage := &session.StorageMemory{}

	client := telegram.NewClient(apiID, apiHash, telegram.Options{
		Resolver: dcs.Plain(dcs.PlainOptions{
			Dial: dc.DialContext,
		}),
		SessionStorage: sessionStorage,
	})

	if err := client.Run(ctx, func(ctx context.Context) error {
		if err := auth.NewFlow(
			auth.Constant("phone", "password", auth.CodeAuthenticatorFunc(codePrompt)),
			auth.SendCodeOptions{},
		).Run(ctx, client.Auth()); err != nil {
			panic(err)
		}

		err := sessionStorage.WriteFile("account.json", 0644)
		if err != nil {
			return err
		}

		return nil
	}); err != nil {
		panic(err)
	}
}

func codePrompt(ctx context.Context, sentCode *tg.AuthSentCode) (string, error) {
	fmt.Print("Enter code: ")
	code, err := bufio.NewReader(os.Stdin).ReadString('\n')
	if err != nil {
		return "", err
	}
	return strings.TrimSpace(code), nil
}

[sadensmol]

you don't have to write session file manually! client writes it. just pass it to:
SessionStorage: &session.FileStorage{Path: "account.json"},