Unfortunately, you can’t use your Xfinity credentials to use the Peacock app. This is true even if you’re getting Peacock Premium free through Xfinity. If you want to watch Peacock through the app on a device other than the Flex or X1, you’ll need a paid subscription.
But, if you don’t mind watching more ads and only half the content, you can set up a Peacock Free account on the Peacock app.