More info: No URL available (#14443)

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>
Co-authored-by: Paulus Schoutsen <balloob@gmail.com>

source/more-info/no-url-available.markdown

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+---
+title: "No URL Available"
+description: "More information on how to resolve the No URL Available message in Home Assistant."
+---
+
+When Home Assistant serves you the "No URL Available" message, you are
+probably trying to set up or configuring an integration that requires your
+account to be linked.
+
+## What happened
+
+The authentication method for linking such an account is called OAuth2, and
+is a modern and more common way of linking systems together, by logging in with
+the provider of the data.
+
+This means Home Assistant will redirect you to the provider to log in. After the
+login is successful, the third-party provider will redirect you back to your
+Home Assistant instance.
+
+For the third-party to be able to redirect/link you back to Home Assistant
+after authentication, Home Assistant needs to tell the third-party application
+about the URL to access Home Assistant on / to redirect back to. Unfortunately,
+if the "No URL Available" error occurs, Home Assistant hasn't been able to figure out
+that URL.
+
+## How to solve it
+
+Home Assistant will try to find the URL you are currently using in your browser
+to use as this redirect URL for the authentication. However, Home Assistant
+is only able to determine this, if the URL is known in the configuration.
+
+There are multiple options to consider:
+
+### Using Nabu Casa Home Assistant Cloud
+
+If you have Nabu Casa's Home Assistant Cloud, the easiest way to resolve this,
+is by visiting your Home Assistant instance from the remote UI URL.
+
+Go to **Configuration** -> **Cloud**.
+
+Visit your instance on the remote URL. Now you can set up the integration as normal, without getting the No URL
+Available message.
+
+### Using your instance by IP address
+
+If you are on your home network, you can visit your instance using the IP
+address of your instance.
+
+For example, `https://192.168.1.2:8123`
+
+Replace the IP with the IP address of your Home Assistant instance. If you
+don't know this, you could also skip this and try the next solution.
+
+Now you can set up the integration as normal, without getting the No URL
+Available message.
+
+### Configuring the instance URL
+
+Another good solution is configuring the Home Assistant instance URL in
+Home Assistant. By letting Home Assistant know about the URL you use to
+access it, Home Assistant will be able to deal with that in a situation
+like this.
+
+Please note, you'll need to enable advanced mode in your user profile in order
+to set this up.
+
+Go to **Configuration** -> **General**.
+
+On this page, two fields that can resolve this issue: "External URL"
+and "Internal URL".
+
+- **Internal URL**: The URL you type in your browser when you are **at home**,
+connected to your home network, e.g., `http://homeassistant.local:8123`
+- **External URL**: The URL you type in your browser when you are **not home**,
+connected to your home network, e.g., `https://example.duckdns.org`
+
+Some additional notes:
+
+- Don't use the Nabu Casa remote UI URL in any of these fields. The remote UI
+  will automatically be handled correctly.
+- If the address you use, is the same for both the "**at home**" and
+  "**not home**" situations, it is recommend to only use the External URL field.
+- The internal URL should preferably not use SSL (have `https://`) in it, as it
+  might cause problems with casting to media devices.
+- If you do not have an external address for your Home Assistant instance,
+  leave that field empty.
+
+After setting the URLs, click save. There is no need to restart Home Assistant
+your changes are applied immediately.
+
+Now you can set up the integration as normal, without getting the No URL
+Available message.