How to use the background population sample?

I have some doubt about Genome STRiP software.
As we all know:To run discovery or genotyping on a single sequenced genome or a small set of genomes, you need to call your data against a background population, such as a set of genomes from the 1000 Genomes Project. The background population does not need to be matched to the target individuals.

My doubt are as follows:
How could I use the background population? If I hava a single sequenced genome sample (named: Sample AA),and I have 20 background sample.I did not find the parameter for the background population. I only know the parameter (- I) for the input bam file for target sample AA. Thank you very much.