[SFTP] Add missing key format note to portal tab

Author: mikewill4

articles/storage/blobs/secure-file-transfer-protocol-support-authorize-access.md

@@ -64,7 +64,10 @@ You can authenticate local users connecting from SFTP clients by using a passwor
    | Use existing key stored in Azure | Use this option if you want to use a public key that is already stored in Azure. To find existing keys in Azure, see [List keys](/azure/virtual-machines/ssh-keys-portal#list-keys). When SFTP clients connect to Azure Blob Storage, those clients need to provide the private key associated with this public key. |
    | Use existing public key | Use this option if you want to upload a public key that is stored outside of Azure. If you don't have a public key, but would like to generate one outside of Azure, see [Generate keys with ssh-keygen](/azure/virtual-machines/linux/create-ssh-keys-detailed#generate-keys-with-ssh-keygen). |
 
-4. Select **Next** to open the **Permissions** tab of the configuration pane.
+   > [!IMPORTANT]
+   > Only OpenSSH formatted public keys are supported. The key that you provide must use this format: `<key type> <key data>`. For example, RSA keys would look similar to this: `ssh-rsa AAAAB3N...`. If your key is in another format, then a tool such as `ssh-keygen` can be used to convert it to OpenSSH format.
+
+5. Select **Next** to open the **Permissions** tab of the configuration pane.
 
 #### [PowerShell](#tab/powershell)
 
@@ -267,4 +270,4 @@ az storage account local-user update --account-name contosoaccount -g contoso-re
 - [Authorize access to Azure Blob Storage from an SSH File Transfer Protocol (SFTP) client](secure-file-transfer-protocol-support-authorize-access.md)
 - [Limitations and known issues with SSH File Transfer Protocol (SFTP) support for Azure Blob Storage](secure-file-transfer-protocol-known-issues.md)
 - [Host keys for SSH File Transfer Protocol (SFTP) support for Azure Blob Storage](secure-file-transfer-protocol-host-keys.md)
-- [SSH File Transfer Protocol (SFTP) performance considerations in Azure Blob storage](secure-file-transfer-protocol-performance.md)
+- [SSH File Transfer Protocol (SFTP) performance considerations in Azure Blob storage](secure-file-transfer-protocol-performance.md)