The art to login to Betfair using APi-NG is not to have login button at all.

Putting a login button is redundant as your app should be able to automatically login when you start it up. Apps should be designed with the least amount of typing and more on just clicking. Must also be able to persists state e.g. if you choose from delayed to live key, the next time you open your app the live key is selected applies as well to login and anything you can think of.