Hi,
Thank you for reaching out. You must follow the example here to set the TargetingSettings. When you update the targeting settings, you must specify an exhaustive list of all targeting settings you want; there is no way to just specify a single new targeting setting. To do this, you must first fetch the existing list of targeting settings before you attempt your update, and make sure you include all the existing ones along with the new one you want to add.
Regards,
Mitchell
Google Ads API Team