Auto add custom field value for new posts?

Hi,

Is there a way to have an automated way to create a custom field name and value for evry new post that is created in certain categories (but not all of them) ?

For Example: When operator of a site will write a new post on a category X or Y, a new custom field name will be created and it will have a predefined value.

I’d be happy to do that not with a plugin, but code it inside the template (maybe with filters or some other function to do it?)

Thanks,
M.