How to define a keyword for automatic keyword expansion on commit

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Git

I need to insert third party licensing/copyright info into all of my source files. To simplify this process, I would like to define keywords that GIT can automatically expand into the full copyright text, similar to keywords like @version.

Is it possible to define my own keywords? If so, what do I need to do?

Or is there an existing keyword which lets me define the text that it will be replaced with?

What I have tried:

I have searched the web, but all hits only show how to use standard keyword expansion, not how to define a new one, or whether it is possible at all.