This KB article uses a free plan as an example, but my free plan does not allow me to create an external Flex service.
Please note that the screenshot referenced above from the KB article is from ngrok and indicates the ngrok account plan type, which is unrelated to the Kinvey account plan type.
Also, as of April 2018 all editions of Kinvey were updated include access to Flex. Please refer to the pricing page for up-to-date indication of features in each edition: https://www.kinvey.com/pricing/
Currently the Flex service is only available to Business and Enterprise subscription plans and is not available with free plans, so the picture is misleading in this sense.
Let me know if you have further questions.