code.js

console.log("hello world");

chrome.webRequest.onBeforeRequest.addListener(
    function(details) {
        console.log(details);
        if (details.url.match(/test/) && details.method=="POST"){
            console.log("hi");
        }

    },
    {urls: ["<all_urls>"]},
    ["blocking"]);

manifest.json

{
  "manifest_version": 2,
  "version":"1",
  "name": "Test Extension",
  "permissions": [
          "webRequest",
          "webRequestBlocking",
          "*://localhost:*/"
        ],
   "background":
        {
            "scripts":["code.js"],
            "persistent": true
        }
}