VBS - Custom Field Validation Using Code

Summary:

Hi,

I want to create a custom validator for one of the fields on the Leads object.

I know that there are some pre-defined validators in VBS, but there's an option to create one using code:

I wasn't able to find any document on how to do this. Is there any document for this?

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