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Connection flow

After your backend has obtained a connect token (typically by exchanging an access token during token exchange), you open the Connect UI so the user can authorize an app. The Connect UI URL includes the app as a path segment and the connect token as a token query parameter. Optionally, pass a theme parameter to apply partner-specific branding to the Connect UI. There are two approaches.

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Option 1: Popup with postMessage

Open the Connect UI URL in a popup window and listen for a message back with the connection ID. 
const ZAPIER_CONNECT_BASE_URL = "https://connect.zapier.com";

function connectApp(token, app, theme) {
  const url = new URL(`${ZAPIER_CONNECT_BASE_URL}/to/${app}`);
  url.searchParams.set("token", token);
  if (theme) url.searchParams.set("theme", theme);

  const popup = window.open(url, "zapier-connect", "width=980,height=700");

  window.addEventListener("message", (event) => {
    if (event.origin !== ZAPIER_CONNECT_BASE_URL) return;

    if (event.data.type === "authenticationSuccess") {
      popup.close();
      onConnected(event.data.authId); // numeric connection ID
    }
    if (event.data.type === "authenticationError") {
      popup.close();
      onError(event.data.error);
    }
  });
}

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Option 2: Redirect

Navigate the current window to the Connect UI URL with a redirect_uri. After the user authorizes, Zapier redirects back to your URL with the result as query parameters. 
const ZAPIER_CONNECT_BASE_URL = "https://connect.zapier.com";

function connectApp(token, app, theme) {
  const url = new URL(`${ZAPIER_CONNECT_BASE_URL}/to/${app}`);
  url.searchParams.set("token", token);
  if (theme) url.searchParams.set("theme", theme);
  url.searchParams.set("redirect_uri", window.location.origin + "/callback");

  window.location.href = url.toString();
}
When Zapier redirects back, your callback URL receives:

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On success

https://your-app.com/callback?authentication_id=12345&service=SlackAPI

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On error

https://your-app.com/callback?error=access_denied&service=SlackAPI
Use authentication_id as the connection identifier with your access token.

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Reconnecting

To reconnect an existing connection (e.g. after it expires), add the reconnect_id parameter to the Connect UI URL. This tells Zapier to re-authorize the same connection instead of creating a new one. The value is the connection’s ID. 
url.searchParams.set("reconnect_id", authenticationId);
Everything else — popup vs redirect, theme, postMessage handling — works exactly the same as a new connection.

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Handling popup close

The user may close the popup before completing the flow. Since no postMessage is sent in that case, poll the popup’s closed property to detect this and reset your UI. 
const poll = setInterval(() => {
  if (popup.closed) {
    clearInterval(poll);
    onCancelled();
  }
}, 500);