[metabagel]

Perhaps there is an indentation on the outside of the detergent dispenser where you are meant to pour a bit of detergent for the pre-wash.

Like in the video: https://youtu.be/DAX2_mPr9W8?si=Njn749InqNCbjhQd&t=822

[Kirby64]

Nope, Bosch 800 series specifically do not have an indentation, and also state in their manual that they do not require any prewash detergent and the prewash does not run with any detergent.