How do I add a specific website to notifications on Safari?

Change Websites preferences in Safari on Mac - Apple Support

Scroll down and read Notifications. Select the checkbox that permits websites to ask your permission to send Notifications, and then load the website. When you see a dialog asking if you want to receive them, you can indicate your consent.

That Mac may have already informed that website it doesn't want them. Two suggestions: Despite the fact it is already listed as "Allow" in Change Websites preferences, remove that website by selecting it and clicking the Remove button. Next time it wants to send you Notifications, Safari should ask.

If that fails to work, remove that website's stored cookies. Refer to Change Privacy preferences in Safari on Mac - Apple Support, under Manage Website Data. Remove cookies and website data for that website, or at your option, all of them.

Great! Thanks for the update.