Sharepoint authenticaion in uwp app

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SharePoint

UWP

i am working in UWP and could not able to get cookies response for sharepoint authentication. Priviously i was worked on windows app i achieved it here but the same code not working for UWP app.
Any help is appreciated.

What I have tried:

private async Task<SPOAuthCookies> GetSPOAuthCookies(SamlSecurityToken stsToken)
    {

        Uri siteUri = this.spSiteUrl;
        Uri wsSigninUrl = new Uri(String.Format("{0}://{1}/{2}", siteUri.Scheme, siteUri.Authority, spowssigninUri));
        var clientHandler = new HttpClientHandler();

        await HttpUtility.SendHttpRequest(
            wsSigninUrl,
            HttpMethod.Post,
            new MemoryStream(stsToken.BinarySecurityToken),
            "application/x-www-form-urlencoded",
            clientHandler);

        SPOAuthCookies spoAuthCookies = new SPOAuthCookies();
        spoAuthCookies.FedAuth = clientHandler.CookieContainer.GetCookies(wsSigninUrl)["FedAuth"].Value;
        spoAuthCookies.RtFA = clientHandler.CookieContainer.GetCookies(wsSigninUrl)["rtFA"].Value;
        spoAuthCookies.Expires = stsToken.Expires;
        spoAuthCookies.Host = wsSigninUrl;

        return spoAuthCookies;
    }

my code breaks from this line
this works fine in windows app

spoAuthCookies.FedAuth = clientHandler.CookieContainer.GetCookies(wsSigninUrl)["FedAuth"].Value;