An employer is required to accommodate only known disabilities. Therefore, it generally is your responsibility as a disabled employee to inform your employer that an accommodation is needed.
If you think you will need a reasonable accommodation in order to participate in the application process or to perform essential job functions, you should inform the employer that an accommodation will be needed, so that you are protected by the ADA if you are not accommodated.
Your employer is under an obligation to keep information about your disability that you disclose confidential. The ADA requires that the results of all medical examinations must be kept confidential and maintained in separate medical files.