So you have to give it a larger set of locations from which it will pick a smaller set to best serve the demand locations.
The input set of candidate locations could have existing facility locations. Some of these can be marker "Required", that is, they have to be in the solution.
Generally you want to give a set of candidate facilities where you can actually build, buy, rent, lease facilities to serve the demand. Not all locations in an area may be suitable or zoned for the type of facility you need to build.