If you're at least a king there's a Decision called "Adopt Special Succession Type" which allows you to adopt it for your primary title at a discount IIRC, plus it gives a +30 opinion bonus with same-culture vassals for 30 years.It's not a realm succession but a title succesion law, so pick a title and click add a law or whatever and it should be there. That's also how you access Scandinavian succesion or saxon elective if you don't already have it
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