Actually now I am able to connect with using this configuration in code
var connSettings = new ConnectionSettings(new Uri("https://elastic-cluster-es-http.default.svc.cluster.local:9200"))
.BasicAuthentication("elastic", "testPassword")
.ThrowExceptions()
.EnableApiVersioningHeader()
.ServerCertificateValidationCallback((o, certificate, chain, errors) => true)
.ServerCertificateValidationCallback(CertificateValidations.AllowAll);