The cdc seems to say the dignosis is more rigorous than some sort of internet self-assessment:
Diagnosing ADHD usually includes a checklist for rating ADHD symptoms and looking at the person's history of behavior and experiences. The provider will determine if ADHD symptoms were present before age 12 years and may ask for permission to gather information from friends and family. A medical and psychological exam may be needed to rule out other health problems that can cause symptoms like ADHD or that are often present with ADHD, such as anxiety, depression, sleep problems, alcohol or substance misuse, or learning disabilities.
Diagnosing ADHD usually includes a checklist for rating ADHD symptoms and looking at the person's history of behavior and experiences. The provider will determine if ADHD symptoms were present before age 12 years and may ask for permission to gather information from friends and family. A medical and psychological exam may be needed to rule out other health problems that can cause symptoms like ADHD or that are often present with ADHD, such as anxiety, depression, sleep problems, alcohol or substance misuse, or learning disabilities.
https://www.cdc.gov/adhd/articles/adhd-across-the-lifetime.h...