Hello there, @matheen-menahub-.
Currently, the option to customize the purchase order form is unavailable. I know how beneficial it is if we're also able to personalize this form like invoices and other sales forms.
I've added this article for your future task about customizing your sales forms: Customise invoices, estimates, and sales receipts in QuickBooks Online. It includes the steps on how to customize custom templates.
Feel free to tag me in the comment below if you have additional questions. I'm always here to help. Take care!