Ok, I solved my problem. I'm not sure if this is the proper way to do this, but anyway.
For someone who is interested in how I do this:
1. Delete all the Bands I don't need in the original product.
2. Use the "subset" tool in the upper toolbox in SNAP, which is under the category "raster".
3. Then the "SNAP - Resampling Required" window will pop up. Clilck yes and you will get a new product with only the one band you wanted.
4. If you safe your geometry as a shapefile you can import it and export the mask pixel without resampling.