HAProxy ReverseProxy through DMZ internet proxy

We have a vendor provided URL https://vendor.domain.com that we need to reverse proxy to from our HAProxy installation. The machine where we have installed the HAProxy is behind a DMZ Proxy. For this configuration to work, we need HAProxy to use the DMZ Proxy proxy.mydomain.com:8555 so that it can reverse proxy the original request to https://vendor.domain.com. Is this possible with HAProxy 1.5.14. How can I make HAProxy itself use an internet proxy?