Crusader Kings III

Actually it still works, you just have to optimise more. Marry a wife with 20+ intrigue, set her to help with court intrigue; and have one of your courtiers marry someone with 20+ intrigue, make them spymaster and set them to supporting your schemes. I also got a Job Done Right perk in the Intrigue tree. With that setup I had an 85% chance of successful kidnapping, even without bribing anyone to join my scheme.

Just to make it clear (I believe it is what you meant, since Eudes Capet starts as a vassal of France): You don't declare a claim war, you create a claimant faction with you as the claimant, then present your ultimatum to your liege.

Due to recent nerfs to the kidnapping scheme, it is tantamount impossible to succesfully kidnap the ruler of a nation when you are already at war, unless you started the scheme before and declare war only just when the event pops up that asks you to do the kidnapping attempt, but before the event is resolved (because you can't declare war on a foreign ruler who is already imprisoned by you).

With faction rebellions like this one here, it is still quite easy though, since technically it's the liege declaring war on the faction when he rejects the demands, so you can first kidnap your liege and then present your demands when he is already safely in your prison cell, giving you an instant 100% win score.