How do I add a custom field or drop down box to the header of a purchase order?

It's nice to have you here in the Community forum, @redtrack80. I appreciate you for sharing the details of your concern about adding custom fields to your purchase order header. I've got some information and procedures to solve your query. 

In QuickBooks Desktop (QBDT), we can only add a textbox field to your purchase order headers by simply adding custom fields to sales forms. To guide you further, please follow the steps below:

1. Go to the Vendors menu and select Vendor Center.
2. Double-click any profile on the list to open and edit it.
3. Choose the Additional Info tab and tap Define Fields.
4. Name your custom field in the Label column.
5. In the Use for column, tap the checkbox to add the custom name field to the vendor profile.
   

After that, let's add this to your purchase order form. Here's how:

1. Create or edit a purchase order.
2. Select the Formatting tab and then Customize Data Layout.
3. In the window, select the tab for the section of the form you want to add your custom field to.
4. From the Header tab, find your custom item field on the list. Select the Screen or Print boxes to add it.
5. Once done, select OK.
   
    

Moreover, please note that adding a field may affect the layout of your form. If you need to, here's how to adjust the layout. Refer to this article for more details about the process: Create and use custom fields in QuickBooks Desktop.

Furthermore, you can visit this article to view how Accounts Payable is used to track the money you owe from your vendors: Accounts Payable workflows in QuickBooks Desktop.

Please keep me updated on how it works for you after following these steps. Please add a reply below if you have other questions or concerns about your purchase orders in QBDT. Have a great day ahead!