You need to change the selection within the textbox.  This is done with the
SelectionStart and SelectionLength properties.  The start is the start
position for selection, the length is the number of characters to be
selected from that point or zero for no selection, just your flashing
cursor.  For example, the following code moves the cursor to just after the
fifth character in the textbox.

           textBox1.SelectionStart = 5;
           textBox1.SelectionLength = 0;