TextBox and MonoGame TextInput - fampat/Myra GitHub Wiki
By default Myra translates Keys to chars in the TextBox widget, which basically limits accepted chars to ASCII only.
Add following code to the Game.LoadContent to make Myra accept any chars by utilizing MonoGame TextInput:
// Inform Myra that external text input is available
// So it stops translating Keys to chars
Desktop.HasExternalTextInput = true;
// Provide that text input
Window.TextInput += (s, a) =>
{
Desktop.OnChar(a.Character);
};