When using WSO2 ESB, there are situations in which you need to talk to an endpoint which sits behind a proxy. In such cases, you should configure the ESB transport sender to forward the messages to the proxy server.
In order to do so, you just need to add the following two parameters as child elements of <transportSender name="http" class="org.apache.synapse.transport.nhttp.HttpCoreNIOSender">
<parameter name="http.proxyHost" locked="false">localhost</parameter>
<parameter name="http.proxyPort" locked="false">8090</parameter>
Note:- Change the proxy port and host according to your environment