Slack notification behind proxy issues

Hi all, please i need help with an issue in slack notification on grafana v6.3.
I have all my dashboard working perfectly and now i am trying to send alert notification using slack. When I do click on the test notification button i see a green popup that’s tell me OK, but the message never come to the channel on slack. After a time reading on internet I found that I have to set the proxy configuration on the file /etc/default/grafana-server. So I applied the following configuration:
https_proxy=‘my ip proxy’
no_proxy=,localhost,::1,‘all ip’s and nets that I don’t to use the proxy’
After restart grafana services the test button works ok but all my dashboards show me the error “bad gateway” and all panels are black. When I do a tcpdump on the server I see that the proxy is applied to destinations that not have to apply.
Could someone tell me if i possible only apply the proxy configuration for slack notification or some workaround to solve this?
Thanks in advance.