Hi Swati,
Are you using the mendix cloud or locally?
Mendix cloud licensed
The the custom settings can be configured in the sprinter portal. If you go to Environmnets > click a containers details > runtime tab.
Locally
This can be configured in the Project module > settings > configurations > edit configuration > custom tab
When defining the custom setting the name will be http.proxyHost
or http.proxyPort
and the value will be the value you want to set.
Hope this helps!