All references can be submitted at any time throughout the process. An email confirming the sending of the form will be sent to your email address. Please note that all references must be submitted prior to the deadline date.
Reference forms from employers are to confirm that you worked at that agency. If your manager no longer works there, then you have a couple of options:
If you can reach them, you can ask them to complete the form based on the time you worked there.
You can ask human resources to complete the form to verify that you worked at that agency.