Adding Spouse to Apple Card

You cannot find that information yet. It sometimes takes a few days to update the Knowledge Based articles on here. I have been searching for them since the words left his mouth with no success. I will continue to look and monitor it and then add it to my User Tip here - Steps to take for applying to Apple Card - Apple Community

I suspect that there will be a software update available soon that unlocks this feature. I saw the event and I am not doubting you. The article you posted says it will be another ten days (May) before it's active so that makes sense.