You'll need to reconfigure the proxy to allow that access, or rewrite the plan to not use it.
While Nic's answer pretty much summarizes the state of affairs, it's worth mentioning that a notable reason for this limited set of options is Bamboo's long standing lack of support for the JVM nonProxyHosts property, so you might watch/vote/comment on the following issue accordingly:
nonProxyHosts
It looks like you're new here. Sign in or register to get started.