PO Approval Workflow - how to making Approval note is mandatory

Hi,

I am configuring PO Approval workflow in SRM. Client requires when rejecting the PO , approvers has to mention the rejection reason.

so we need to make it approval note is mandatory when rejcting the PO.

Can you please tell me how this can be done in SRM. Is it possible in SRM?

Thanks & Regards,

Sankar.