As someone under 18, you may have an account, I believe both checking and savings, with a parent joint on the account, at Banks and credit unions. Are you sure you don’t have one? Grandparents” birthday checks or transfers to an account with your name, with a parent also in the account to take care of depositing when you were little? My parents had a savings account in my name with my dad on it. That’s where all gift checks and cash gifts went. When I went off to college and turned 18 there, Dad took himself off the account. My parents had taught me about budgeting early on, and how to endorse and write paper checks (then), reconcile monthly account balances and manage my own bank accounts.
When your name is on an account, you then have an account number and bank/CU routing number (unique electronic identifier) to put on your military Direct Deposit form.
You can start out with hometown bank and later close accounts and transfer balances to another bank or CU, as many do. But - you have to tell the govt where to deposit your pay!
USMA does something with PenFed credit union that I don’t know much about. I am mostly familiar with USNA, which doesn’t force you to have an account with any particular financial institution. Perhaps some USMA current cadets, recent grads or other alumni or current parents can clarify.