How to prevent image from rendering to screen but seen by camera

I'm developing an Augmented reality app using ARFoundation. I'm using NatCorder to record video. I want to add watermark for the video. I tried merging watermark before saving the frame but it is useless because the framerate drops to ~10 fps. So I'm planning to place the watermark logo in front of the camera, so it will be automatically added to the video. But the problem is the watermark is visible during recording and I don't want this. I want to prevent it from rendering to the screen but it should be seen by the camera.

Thanks in advance!