Slack webclient communication through proxy fails to verify slack.com certificate even though rejectUnauthorized: false is used in webclient agent

[w7089]

I've raised this issue at Slack web client. It turned out to be an issue with https-agent.

I tried suggestions in this issue. However, none seemed to help and i'm still getting the same issue. Is there a workaround and a way to pass rejectUnauthorized: false somehow to the agent so that the cert validation of the proxied server won't happen?