Hi Zia,
Unfortunately, you cannot use this option at campaign level. To set this at an ad group level, you can use the
targetAll field in the TargetingSettingDetail. If you set the value of this field as true, the audience is set to Bid Only whereas setting the value of this field as false would set the audience to Target & Bid.
Hope this helps. Please feel free to revert in case you have more questions.
Regards,
Vishal, AdWords API Team