My question is, how do I apply the code to the custom field so the auto numbering works in the field I need it to work in? I am lost here. Please help
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I see the example code-shared by Christian, where the code he shared is applied to the post type "project: and the custom field that holds the number is "project-number".
If the field to which you want to store the auto generated ID is not "project-number" then you modify the custom field name "project-number" with your original custom field name with the code he shared.
If you need any help, please feel free to ask further help if required.