Taking USABO at a Host School

Hey guys! I’m interested in taking the USABO exam next year (2021) and it’s almost guaranteed that my school won’t participate, so I’ll have to take it at another host school or authorized USABO test center. Can anyone with experience of taking a national science competition such as the USABO, USACO, USAPhO, and AMC 10/12 describe what it’s like, taking the exam at a host school? I’m a Massachusetts resident, and what’s the chance that a host school will be successfully identified for me? How would I know my score, whether I qualified for the next round, etc.?