Checkout http://appium.readthedocs.io/en/stable/en/writing-running-appium/appium-bindings/.
It is a great reference for your language specific syntax.
I believe that code you need would be something like:
driver.SetContext(“WEBVIEW_COM”);
I don’t believe you need to provide the .MYAPPNAME that you have in your original code, but as my appium tests are written in Ruby, I’m not 100% sure.
Also, make sure that your webview is fully loaded before attempting to interact with your login field.
Good luck and hope that helps!