Following steps may resolve your problem
1. Declare a textbox in your form as given below
private System.Windows.Forms.TextBox txtCurrentBox;
2. Add Enter event handler for every text box like below
private void textBox1_Enter(object sender, EventArgs e)
{
txtCurrentBox = sender as TextBox;
}
3. Add new function like given below.
private void TypeToTextBox(object sender)
{
Button b = sender as Button;
if (txtCurrentBox != null)
{
txtCurrentBox.Text += b.Text;
}
}
4. On click of every keyboard button call this function as given below
private void btn_b_Click(object sender, EventArgs e)
{
TypeToTextBox(sender);
}