docs: clarify ngrok instructions around domains

openhands/usage/agent-canvas/backend-setup/vm.mdx

@@ -16,7 +16,7 @@ Agent Canvas supports two VM runtime modes and several ways to reach them:
 | Setup | Start Command | How You Use It |
 |-------|---------------|----------------|
 | **Backend only** | `agent-canvas --backend-only --public` | Run only the agent server on the VM. Start `agent-canvas --frontend-only` on your laptop and add the VM URL in **Manage Backends**. |
-| **Backend only + ngrok** | `agent-canvas --backend-only --public` and `ngrok http 8000` | Use an ngrok URL as the backend URL. Do not add ngrok OAuth for this mode; rely on `LOCAL_BACKEND_API_KEY` plus a private or temporary URL. |
+| **Backend only + ngrok** | `agent-canvas --backend-only --public` and `ngrok http 8000` | Use your ngrok domain as the backend URL. Do not add ngrok OAuth for this mode; rely on `LOCAL_BACKEND_API_KEY`. |
 | **Full Canvas** | `agent-canvas --public` | Serve both the Agent Canvas UI and the backend from the VM. Open the VM, reverse proxy, or ngrok URL in a browser. |
 | **Full Canvas + ngrok OAuth** | `agent-canvas --public` and `ngrok http 8000 --traffic-policy-file ~/policy.yml` | Protect the full Canvas URL with an ngrok login policy before users reach Agent Canvas. |
 
@@ -41,7 +41,7 @@ Agent Canvas requires:
 - [Node.js](https://nodejs.org/en/download) 22.12 or later, including `npm`.
 - [`uv`](https://docs.astral.sh/uv/getting-started/installation/) for the agent server runtime.
 - `git` and `curl`.
-- Optional: [`ngrok`](https://ngrok.com/download) for a temporary public URL.
+- Optional: [`ngrok`](https://ngrok.com/download) for a public URL on a free ngrok domain or your own custom domain.
 - Optional: `tmux` to keep Agent Canvas and ngrok running after disconnecting from SSH.
 
 ### Ubuntu 22.04 / 24.04
@@ -135,7 +135,7 @@ Use a high-entropy secret. You will enter this key in Agent Canvas when connecti
     3. Enter a name, such as `my-vm`.
     4. Enter the **Host / Base URL**:
        - `http://localhost:8000` if you use an SSH tunnel.
-       - The `https://...ngrok-free.app` URL if you use ngrok.
+       - Your ngrok domain (for example `https://your-domain.ngrok-free.app`) if you use ngrok.
        - Your reverse proxy URL if you use nginx or another proxy.
     5. Enter the `LOCAL_BACKEND_API_KEY` from the VM.
     6. Save and select the backend.
@@ -191,19 +191,34 @@ Then use `http://localhost:8000` as the backend URL in **Manage Backends**.
 
 ### Option B: ngrok Without OAuth
 
-Use ngrok without OAuth only for temporary testing or personal access. Keep `--public` enabled and use a strong `LOCAL_BACKEND_API_KEY`.
+Use ngrok without OAuth for personal access or a small, trusted backend. Keep `--public` enabled and use a strong `LOCAL_BACKEND_API_KEY`.
+
+Every ngrok account—including the free plan—comes with a free static domain that looks like `your-domain.ngrok-free.app`. It stays the same across restarts, so `ngrok http 8000` starts on it by default and you can save the URL once and keep reusing it. You can view your domain on the [**Domains**](https://dashboard.ngrok.com/domains) page of the ngrok dashboard.
 
 On the VM, in a second terminal or tmux pane:
 
 ```bash
 ngrok http 8000
 ```
 
-Use the `https://...ngrok-free.app` forwarding URL:
+Use the forwarding URL:
 
 - Backend-only mode: enter it as the **Host / Base URL** in **Manage Backends**.
 - Full Canvas mode: open it directly in your browser.
 
+#### Use Your Own Domain
+
+To run on a domain you choose instead of the default, pass it with `--url`:
+
+```bash
+ngrok http 8000 --url https://your-canvas.ngrok.app
+```
+
+What you can use depends on your ngrok plan:
+
+- **Hobbyist plan:** an ngrok-branded domain such as `your-canvas.ngrok.app`.
+- **Pay-as-you-go plan:** your own custom domain such as `canvas.acme.com`.
+
 ### Option C: ngrok With Google OAuth
 
 Use ngrok OAuth with **full Canvas** deployments when the ngrok URL may be reachable by a team or a broader audience. OAuth is an additional gate in front of Agent Canvas; it does not replace `LOCAL_BACKEND_API_KEY`.
@@ -235,6 +250,10 @@ Start ngrok with the traffic policy:
 ngrok http 8000 --traffic-policy-file ~/policy.yml
 ```
 
+<Note>
+  To run OAuth on a domain you choose, add `--url` as shown in [Use Your Own Domain](#use-your-own-domain).
+</Note>
+
 To run full Canvas and ngrok side by side in tmux:
 
 ```bash