403 Error - Thats an error. Error: disallowed_useragent

In my case I was using native web view to login with Google, I find out the way that you should provide user agent to webview it was worked for me. Try below code I am sure it will worked.

Add the code in application didFinishLaunchingWithOptions

Objective C

 NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:@"Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36", @"UserAgent", nil]; [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];

Swift 3.0

let dictionary = NSDictionary(object: "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36", forKey: "UserAgent" as NSCopying)UserDefaults.standard.register(defaults: dictionary)

<preference name="OverrideUserAgent" value="Mozilla/5.0 Google" />

I also face this issue on my cordova project. You can try this: Just add this to your config.xml, worked for me.

The short answer is that Google has updated its security restrictions for OAuth flow. They are not going to allow native web-views to initiate OAuth flows, but rather are encouraging people to use the OS browsers to do so. In your case, you'll probably have to wait for the Google calendar SDK to update their code to obey the newly recommended flow. More information is available in the Google blog

EDIT : I have tried to create a cross platform plugin which wraps around the native Google sign-in SDKs for using in a Xamarin forms app. More information can be found here