I have a sample code from xamarin.com:
How to click the Button and take a photo without preview? Maybe you have an articles, or videos. I'm new at Xamarin, so it will be awesome, if i'll get as more details as possible.
This article will get you started with a stream of the camera to a surface in your app. It shows you how to get to the Camera class, which in turn has a TakePicture method that you could use for your problem.
Please also take a look at this answer on SO on a similar question (native Android, but the same concepts apply), which suggest showing the preview on a 1x1 surface because you can't really prevent the preview on Android as far as I know.
PS: don't forget to add the correct permissions in your manifest for Camera access.