Pay on Account feature enabled only for specific customers in Shopify.

Firstly, I am looking to create a feature in our Shopify store where upon checkout customer can select an option to pay on account and leave their Purchase Order Number (see screenshot attached). The entered PO Number should appear in order notes (see the same screenshot attached). Secondly, and I think this requires a custom App, I need the possibility to enable/disable this Pay on Account feature only for selected customers anywhere at our Shopify’s backend. I will leave it up to you how to create and design it, all I need is to have this feature. I just got a Shopfy Plus subscription, so you have control of the [login to view URL] files. Thank you and I look forward to your bids. P.S. Please quote 'ACCOUNT' if you've read this project description, so I can filter out freelancers who place blind bids without knowing what they bid for.