Make Purchase Order (Po#) Field Mandatory in Payment Information Page of the Standard Web Store Checkout

A. Disable the existing 'Purchase Order' field displayed on the Payment page under Setup > Site Builder > Set Up Web Site > Shopping tab > Payments Page: Display Purchase Order Field on Payment Info Page = un-check then Save.
B. Create a NEW Transaction Body Field 

    1. Navigate to Setup > Customization > Transaction Body Fields > New

    2. Enter and set the following details in the new transaction body field to create a new MANDATORY PO# field 
           Label: PO#, Type: Free-Form Text, Store Value: T 
           Applies To tab: Purchase (checkbox): T, Sale (checkbox): T, Web Store (checkbox): T 
           Display tab:  Display Type: Normal, Subtab: Main 
           Validation & Defaulting tab: Mandatory (checkbox): T

 C. Customize transaction form preferred on the Web Store (Setup > Site Builder > Set Up Web Site > Shopping tab > Checkout Preferences)
      Navigate to Setup > Customization > Transaction Forms > edit/customize Sales Order Transaction form
      Disable default PO# field by navigating to Screen Fields tab > Main tab > 'Ref.No.' Show = F > Save