I have created a trivial application to test the following functionality. When my activity launches, it needs to be launched with the softkeyboard open.
Solution for Xamarin developers (_digit1 == EditText):
var focussed = _digit1.RequestFocus();
if (focussed)
{
Window.SetSoftInputMode(SoftInput.StateAlwaysVisible);
var imm = (InputMethodManager)GetSystemService(InputMethodService);
imm.ToggleSoftInput(ShowFlags.Forced, 0);
}