code: private void addProxyConfig(RequestConfig.Builder requestConfigBuilder, HttpClientSettings settings) {
          if (settings.isProxyEnabled() && settings.isAuthenticatedProxy() && settings.getProxyAuthenticationMethods() != null) {
              List<String> apacheAuthenticationSchemes = new ArrayList<String>();
  
              for (ProxyAuthenticationMethod authenticationMethod : settings.getProxyAuthenticationMethods()) {
                  apacheAuthenticationSchemes.add(toApacheAuthenticationScheme(authenticationMethod));
              }
  
              requestConfigBuilder.setProxyPreferredAuthSchemes(apacheAuthenticationSchemes);
          }
      }
docs: Update the provided request configuration builder to specify the proxy authentication schemes that should be used when
  authenticating against the HTTP proxy.
  
  @see ClientConfiguration#setProxyAuthenticationMethods(List)