If you wish to take an AP exam for a course not offered at El Capitan, search the AP Course Ledger to find a school that offers it. When you find one, call that school to ask for the name of the AP Coordinator. Email that person and ask about being included in their test administration.
If the answer is no, keep looking on the Course Ledger and contacting schools. Updated lists of which schools are offering the course will be posted on the Course Ledger in November. If you find one after the order deadline, a school may still accept your test order with a late fee.
If the answer is yes, that AP Coordinator will need to provide a code for an Exam-Only section (class) on the College Board website, where you will create an account if necessary, log in, and indicate that yes, you intend to take the test. They will also need to collect your payment and any required forms from you.