TrustFailure: Could not establish trust relationship for the SSL/TLS secure channel, Dynamics 365 online

webRequest = WebRequest.Create(p_Url);
webRequest.Method = WebRequestMethods.Http.Post;
webRequest.Headers[HttpRequestHeader.Authorization.ToString()] = "Bearer " + p_ApiKey;

byte[] byteArray = Encoding.UTF8.GetBytes(p_Json);
webRequest.ContentType = "application/json";
webRequest.ContentLength = byteArray.Length;

Stream dataStream = webRequest.GetRequestStream();
dataStream.Write(byteArray, 0, byteArray.Length);
dataStream.Close();

string searchTokenResponse;
using (IWebResponse response = webRequest.GetResponse()) {
    using (Stream dataStream = response.GetResponseStream()) {
        using (StreamReader streamReader = new StreamReader(dataStream)) {
            searchTokenResponse = streamReader.ReadToEnd();
        }
    }
}