Many people delay asking for support because they are waiting for a diagnosis. This can be common for ADHD, autism, dyslexia, dyspraxia and mental health conditions.
The important issue is the impact on work. Explain how your access need affects your role, travel to work or interview communication. Evidence can help where available, but a lack of diagnosis should not stop you from checking your options.
Use plain examples. For example: “long written instructions take much longer to process and I miss actions when several requests are combined.”