diff --git a/action.yml b/action.yml
index dc97954..18ff0e6 100644
--- a/action.yml
+++ b/action.yml
@@ -7,7 +7,10 @@ inputs:
   repo:
     description: GitHub owner and repository names in the form of "owner/repo".
     required: false
+  url:
+    description: The URL in the browser pane of CodeSandbox. Eg. "https://fe8lf.sse.codesandbox.io/"
+    required: false
 runs:
   using: docker
   image: Dockerfile
-  args: ["${{ inputs.repo }}"]
+  args: ["--url", "${{ inputs.url }}", "${{ inputs.repo }}"]
diff --git a/index.js b/index.js
index 4292207..7bfdd9c 100644
--- a/index.js
+++ b/index.js
@@ -1,8 +1,6 @@
 // @ts-check
 const fs = require("fs");
-const arg = require("arg");
 const puppeteer = require("puppeteer");
-const { GITHUB_REPOSITORY } = process.env;
 
 function inDocker() {
   return new Promise(resolve =>
@@ -10,33 +8,30 @@ function inDocker() {
   );
 }
 
-async function main() {
-  const args = arg({
-    "-h": "--help",
-    "--help": Boolean
-  });
-
-  if (args["--help"]) {
-    console.log(
-      [
-        `Usage: ${process.argv0} [options] repo`,
-        "Options:",
-        " -h, --help  print command line options"
-      ].join("\n")
-    );
-    return;
-  }
-
-  const repo = args._[0] || GITHUB_REPOSITORY;
-  if (repo == null) {
-    throw new Error(
-      "sync-codesandbox requires a GITHUB_REPOSITORY environment variable like 'owner/repo'"
-    );
-  }
+/**
+ * @param {import("puppeteer").Page} page
+ * @param {URL} url
+ */
+async function initializeSandbox(page, url) {
+  const loadingTitle = "Sandbox - CodeSandbox";
+  const loadingText = "Initializing Sandbox Container";
+  await page.goto(url.href);
+  await page.waitForFunction(
+    `document.title !== "${loadingTitle}" && !document.body.textContent.includes("${loadingText}")`
+  );
+}
 
+/**
+ * @param {string} repo GitHub owner and repository names in the form of "owner/repo".
+ * @param {URL} url The URL in the browser pane of CodeSandbox.
+ */
+async function main(repo, url) {
   const options = (await inDocker()) ? { args: ["--no-sandbox"] } : {};
   const browser = await puppeteer.launch(options);
   const page = await browser.newPage();
+
+  if (url) await initializeSandbox(page, url);
+
   const response = await page.goto(`https://codesandbox.io/s/github/${repo}`);
   await browser.close();
 
diff --git a/sync-codesandbox b/sync-codesandbox
index f7ee670..7754383 100755
--- a/sync-codesandbox
+++ b/sync-codesandbox
@@ -1,8 +1,45 @@
 #!/usr/bin/env node
 
+const args = require("arg")({
+  "-h": "--help",
+  "--help": Boolean,
+  "--url": String
+});
+
+if (args["--help"]) {
+  console.log(
+    [
+      "Usage: sync-codesandbox [options] repo",
+      " repo        GitHub owner and repository names.",
+      "             eg. owner/repo",
+      "Options:",
+      " -h, --help  Print command line options.",
+      " --url       The URL in the browser pane of CodeSandbox.",
+      "             eg. https://fe8lf.sse.codesandbox.io/"
+    ].join("\n")
+  );
+  return;
+}
+
+const repo = args._[0] || process.env.GITHUB_REPOSITORY;
+if (!repo) {
+  throw new Error(
+    "sync-codesandbox requires a GITHUB_REPOSITORY environment variable like 'owner/repo'"
+  );
+}
+
+const url =
+  args["--url"] &&
+  new URL(
+    [
+      args["--url"].match(/^\s*https?:\/\//) ? "" : "https://",
+      args["--url"].trim()
+    ].join("")
+  );
+
 // NOTE: Unhandled promise rejection terminates Node.js process with non-zero exit code.
 process.on("unhandledRejection", event => {
   throw event;
 });
 
-require("./index.js")();
+require("./index.js")(repo, url);