If you (unlikely) need to update all the textboxes with the same button then list all of them in your handler, e.g
...
Search_Form.tb.Text += b.Text;
Search_Form.tb1.Text += b.Text;
...
On the other hand, if you need to update just the 'currently selected' textbox then you should implement a mechanism for identifying it. For instance you might assign to the
curSelTextBox
variable a reference to the last text box that got the input focus. Then, in your event handler use:
...
curSelTextBox.Text += b.Text;