Default button property in winform app

It is called AcceptButton now on the form; set that to the button that will be the default button.
Refer to Form.AcceptButton Property

In addition to Form.AcceptButton property the "OK" button must have the TabOrder property set to 0 and all other controls within the form should have a TabOrder >0.

This can be done using a form resouce contruction kit or by code eg. buttonOK.TabOrder = 0;

I think you want the "AcceptButton" property at the FORM level... That will expose a combobox of available controls on your form, then select your "button" you want to use as the "Default" button on enter.