Using declarativeNetRequest on Proxy

Question

Scenario: Browser -> proxyurl.com -> target.com

Is it possible to modify the headers sent to the proxy?
It appears that the urlFilter refers to the target.com.

const removeRuleIds = [1];
const addRules = [
  {
    id: 1,
    priority: 1,
    action: {
      type: 'modifyHeaders',
      responseHeaders: [
        {
          header: 'header-1',
          operation: 'set',
          value: 'header1-added'
        }
      ]
    },
    condition: {urlFilter: '||proxyurl.com' }
  }
];

chrome.declarativeNetRequest.updateDynamicRules({removeRuleIds, addRules});