Cómo configurar ZohoDesk e Identity Server?

Tengo un problema con esta conexión.
Tengo un método Login(). El método captura los datos del usuario con un formulario. Pero no sé cómo obtener el token y luego enviarlo a Zoho Desk.
La configuración de mis clientes:

new Client
{
    ClientId = "zoho.com",
    ClientName = "Zoho Desk",
    ClientSecrets =
    {
        new Secret("secret".Sha256())
    },
    AllowedGrantTypes = GrantTypes.HybridAndClientCredentials,
    AccessTokenType = AccessTokenType.Reference,
    AlwaysIncludeUserClaimsInIdToken = true,
    AlwaysSendClientClaims = true,
    Enabled = true,
    RequireConsent = false,
    AllowOfflineAccess = true,
    UpdateAccessTokenClaimsOnRefresh = true,
    ClientUri = "zoho.com",
    BackChannelLogoutUri = $"{coreSettings.ZohoSamlRequestUrl}",
    PostLogoutRedirectUris = postLogoutRedirectUris,
    RedirectUris = {$"{coreSettings.ZohoSamlResponseUrl}"},
    RefreshTokenUsage = TokenUsage.ReUse,
    RefreshTokenExpiration = TokenExpiration.Absolute,
    IdentityTokenLifetime = 300,
    AuthorizationCodeLifetime = 300,
    AccessTokenLifetime = 3600,
    AbsoluteRefreshTokenLifetime = coreSettings.AbsoluteRefreshTokenLifetimeInSeconds,
    AllowedScopes =
    {
        StandardScopes.OpenId,
        StandardScopes.Email,
        StandardScopes.Profile
    }
}

Recibo el mensaje “Invalid SAML Response received.” después de redirigir a https://accounts.zohoportal.com/accounts/csamlresponse/xxxxxxx.