I am not sure what you mean by inserted it in the design. I do not show that SelectionStart is a valid property that can be set within the Windows Forms designer.
I was setting it in the new method directly after the InitializeComponent was invoked
method-id NEW.
procedure division.
invoke self::InitializeComponent
set textBox1::Text to "ABC"
set textBox1::SelectionStart to 0
set textBox2::Text to "12345"
set textBox2::SelectionStart to 0
goback.
end method.