Congratulations on your new iPhone 8 Plus! I'm sure you're excited to use all of its great features, including the Camera. From your post, I understand you've noticed a speck of dust or something under the camera lens. I'm happy to help!
The best recommendation I can make if you feel there is dust or other debris under the lens would be to take the iPhone into an Apple Store or Apple Authorized Service Provider for an inspection. A Genius or Apple-certified technician will be able to determine the best course of action to rectify the situation.
You can find locations, get contact information, and set reservations here: Apple - Find Locations .
You are also welcome to reach out to Apple Support directly for additional information and to review your options. You can do that here: Contact Apple Support .