Docs: Update Zendesk Custom OAuth (#359)

Author: EricGustin

pages/home/auth-providers/zendesk.mdx

@@ -23,21 +23,24 @@ This auth provider is used by:
 ## Create a Zendesk app
 
 ### Additional guides
-The following two guides from Zendesk will be helpful additional information as you progress through this guide:
+The following three guides from Zendesk will be helpful additional information as you progress through this guide:
 1. [Using OAuth authentication with your application](https://support.zendesk.com/hc/en-us/articles/4408845965210-Using-OAuth-authentication-with-your-application)
 2. [Set up a global OAuth client](https://developer.zendesk.com/documentation/marketplace/building-a-marketplace-app/set-up-a-global-oauth-client/)
+3. [Getting a trial or sponsored account for development](https://developer.zendesk.com/documentation/api-basics/getting-started/getting-a-trial-or-sponsored-account-for-development/)
 
 ### Creating a Zendesk app for Arcade
 1. Create your Organization in the [Zendesk Marketplace portal](https://apps.zendesk.com/).
 1. Create a Zendesk support account at https://www.zendesk.com/login . If you need a global OAuth client, then the subdomain MUST begin with "d3v-". You will need a global OAuth client if your app will use integrations/tools for multiple customers with their own Zendesk instances (multiple subdomains).
 1. In [the Admin Center](https://support.zendesk.com/hc/en-us/articles/4581766374554#topic_hfg_dyz_1hb), click "Apps and integrations" in the sidebar, then select APIs > OAuth clients > Add OAuth client.
-    * Ensure your identifier is prefixed with "zdg-" If you will need a global OAuth client.
+    * Ensure your identifier is prefixed with "zdg-" if you will need a global OAuth client.
     * Select "Public" for "Client kind".
     * Use `https://cloud.arcade.dev/api/v1/oauth/callback` as your "Redirect URL".
 1. Copy and store your identifier for later. This will be your **Client ID**.
 1. Copy and store your generated secret for later. This will be your **Client Secret**.
 1. (Only for Global OAuth client) Request a global OAuth client.
-    * Sign into the [Zendesk Marketplace portal](https://apps.zendesk.com/)
+    * First, you will need to request a sponsored account and wait for approval from Zendesk. You can request a sponsored account [here](https://developer.zendesk.com/documentation/api-basics/getting-started/getting-a-trial-or-sponsored-account-for-development/#requesting-a-sponsored-test-account).
+    * When filling out the sponsored account request form, ensure you select "App Developer / ISV" as your Developer Type.
+    * After you have a sponsored account, sign into the [Zendesk Marketplace portal](https://apps.zendesk.com/)
     * Organization > Global OAuth Request and fill out the form. Zendesk will typically review your request within 1 week.