Typing through keyboard in different textboxes

Question

i have several text boxes on a form and i want a keyboard in my windows application by which a user could type in different textboxes. 
i made key board using many button and every button behind this code.

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Button b = sender as Button;        Search_Form.tb.Text += b.Text;

but this is working for only one textbox.
a genral keyboard i require by which a user could type in any textbox