Best way to play with Form Designer

Question

Hello,
I am working with windows form since a few days, but having some question about the good way to play with the windows form designer. If I use "external" class with object data list and I want to use use them with a ComboBox for example:

<code>

 this.comboBox1.FormattingEnabled = true;

            /*this.comboBox1.Items.AddRange(new object[] {

            "01",

            "02",

            "03"

            });

             * */

            Data myData = new Data();

            this.comboBox1.Items.AddRange(myData.m_data);
</code>
then If I add modification to the form with the Designer all my added code  is reset. Does that mean that If I insert code to the Windows Form Designer generated code I can't no more use the Designer ? or is there any secret tricks ? or is it better to not use the designer ?
thanks for you help.