If it’s like the full version of Wavelab, make a time selection of the background noise you want to use, then Copy, then go to menu make File/New/Audiofile, then Paste, then save that as a separate audio file called “noise” or something. Then go back to the Silence dialog and select that new background noise file you just made in the file box under the Background Noise radio button.
I agree the manual should suggest something like this, because normally you’d take a noise sample from the file you’re working on.