Hmmm worked for me.
I did claim throne on Spain. The guide goes into detail, but there is an event called "Spain a Hapsburg Throne". When Castile/Spain has no heir and you are both rivalling France, the event can fire and in fact usually fires.
After they have your dynasty there are a few different ways to go about it. When they had no heir I broke alliance, claimed throne, and truce broke. (This isn't the best way to do it lol). Fully diplo and religious plus other modifiers made the stab hits not so bad.
I did claim throne on Spain. The guide goes into detail, but there is an event called "Spain a Hapsburg Throne". When Castile/Spain has no heir and you are both rivalling France, the event can fire and in fact usually fires.
After they have your dynasty there are a few different ways to go about it. When they had no heir I broke alliance, claimed throne, and truce broke. (This isn't the best way to do it lol). Fully diplo and religious plus other modifiers made the stab hits not so bad.