Implementation according to your answer:
public AdGroupExtensionSettingReturnValue CreateAccountCalloutExtension(long adGroupId, string calloutText)
{
AdGroupExtensionSetting adGroupExtensionSetting = new AdGroupExtensionSetting
{
adGroupId = adGroupId,
extensionType = FeedType.CALLOUT,
extensionSetting = new ExtensionSetting()
{
extensions = new ExtensionFeedItem[]
{
new CalloutFeedItem() {calloutText = calloutText}
}
}
};
var operation = new AdGroupExtensionSettingOperation()
{
operand = adGroupExtensionSetting,
@operator = Operator.ADD
};
return _adGroupExtensionSettingService.mutate(new[] { operation });