Hello,
If you want to show item number from Item List to Purchase Order list then you must create extension for that.
Big Question is what if the purchase order have more than 1 item number? In that scenario do you want to show first item number or all item numbers with | (Pipe). Anyway I am adding the code for multiple as well single item number.
Code for single Item Number:

Solution:

Code for Multiple Item Numbers:​​​​​​​

Solution:

Signed, Sealed and Delivered.
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Thanks & Regards,
Ankit Shah(MCT-Microsoft Certified Trainer || Dynamics 365/Business Development Manager)
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