Sorry, but it is not allowed in UWP due to some APIs restriction to be called only by user interaction.
Instead, you can programmatically write text to the textboxes like:
// To simulate key 'A' and 'B'
Textbox1.Text += 'A';
Textbox1.Text += 'B';
// To simulate backspace if Textbox contains any character
if (Textbox1.Text.Length > 0)
Textbox1.Text = Textbox1.Text.Remove(Textbox1.Text.Length - 1);
Problem with this snippet is, you can't simulate special key like ALT, CTRL, F1-F12, Shift and WinKey.