How do I add widgets to Notification screen?

I'm very sorry. I thought (incorrectly) you were asking about Today View, which is what you see when you swipe right from the left edge of your home screen.

View and organize Today View on iPhone - Apple Support

Notification Screen widgets are governed at Settings > Notifications > [the app in question]

Scroll all the way to the bottom of the screen and press the "Edit" button there.

Glad that cleared things up.

Enjoy your iPhone.