Add Part Details Popup: How can we add new custom field

Summary:

On Service Request detail page we have enable Parts subtab. To add part, a popup appear to select Service Activity code and item details.

We have business requirement to add custom field on add part popup. We have create Custom Layout for Details Layout, this consist subtabs: parts. On selection parts subtab it shows message This is not extensible. Kindly guide me, how we can meet this requirement.

Content (required):

Add parts page on which new custom field needs to be added .

Details Layout: Custom Layout

Version (include the version you are using, if applicable):

21C

Code Snippet (add any code snippets that support your topic, if applicable):