It depends on what country you are in. I am assuming most Western countries and even others, have a similar process than the US. You must have a diagnosis from a qualified medical professional, such as a diagnostic specialist, after being referred by your psychologist or doctor. They will conduct a battery of tests, interview your family and others, and determine whether you have a disability. In the US, to show that you are disabled you must be shown to have significant impairments in one or more major life activities, like thinking, speaking and socializing. The Federal govt. (of the United States) recognizes certain diagnoses as automatically constituting a disability under the ADA, and ASD is among these. It is an expensive process, and time consuming, and invasive of your privacy, but this is what I had to do.
If you already have a diagnosis, check their website. They probably have a process.
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Autism