It is very difficult to get this accommodation unless your child can document that they have needed extra time in the past in order to complete their every day school work - tests and such. If they don’t have an IEP or testing stating they need extra time, talk to the counselors in charge of special education at your school and they can advise you - in many schools these are the folks who file the paperwork for you.
Also be aware that extra time is not always helpful, some adhd kids for example find the extended test time unbearable.