you apply/interview for those positions along with everyone else, you don't get preferential treatment because you are a scholar...along those lines, it is illegal for a site to pay you less because you are a scholar...they must pay you the going rate for the area.
There are two phases to the "placement"
First, you actively search and land a job (i think this is around the middle of your last year of residency) and the NHSC gives it their stamp of approval by "assigning/placing" you there.
Phase 2 begins if come May (i think) you don't have a job lined up yet. The NHSC will meet with you and go over ideas & areas you would like (and not like) to work and they will do their best to place you somewhere you will be happy. The whole goal of this process is to get you somewhere you WANT to stay. That way, they are one physician closer to not being underserved. It does the area/program no good if after 2, 3 or four years you are dying to get out.