Update media-services-v3-cli-access-api-include.md

You might want to tweak the changes as needed - one more step needed if you have access to many subscription

Author: anilmur

includes/media-services-v3-cli-access-api-include.md

@@ -16,6 +16,12 @@ To connect to Azure Media Services APIs, you use the Azure AD service principal
 
 Before running the script, you should replace the `amsaccount` and `amsResourceGroup` with the names you chose when creating these resources. `amsaccount` is the name of the Azure Media Services account where to attach the service principal.
 
+If you have access to multiple subscriptions, then you should first set the active subscription to that under which the Media Service account was created.
+
+```azurecli
+az account set --subscription subscriptionId
+```
+
 The following command returns a `json` output:
 
 ```azurecli