Given the mention of NHS on the sign, this is focused on the UK.
Step 1. Note down the various reasons you think you might have adhd. Also include times that it has actively had a negative effect.
Step 2. Make a GP appointment.
Step 3. At your appointment, explain that you would like a referral for a mental health assessment, since you believe you have adhd. The note previously can either act as a prompt for yourself or just give them to the doctor. They were mostly to crystalise your thoughts and stop you going blank at the appointment.
Congratulations, you are now on the (very slow) path to a diagnosis. There are methods to speed it up, but even if you just passively follow instructions, you should get there.
Different countries will have slightly different systems, but the broad approach should still work. For our American friends, you have our commiserations.
In germany call 116117, tell them you want a consultation with a psychiatrist. Tell said psychiatrist what you think you have, eventually you'll get tested and the procedure continues. Took me about 5-6 Months due to scheduling, but eventually it works.
When I tried it, 116117 didn't have appointments in my area, so good luck and you're on your own.
I only got an appointment thanks to the good relations of someone I know.
Now, after being diagnosed, I'm lucky to be medicated. But I'm stuck not getting the therapy I'd probably need dearly.
"this didn't happen to me therefore it happens to nobody"
When I was in 3rd grade they pumped me so full of Concerta that I immediately became silent in class, so silent I lost most of my friends that knew me as the talkative class clown, and stunted my social ability lasting to this day 25 years later. Not saying this is the norm, just saying it is indeed a possibility
I think the best way to know what you need is by being you.
One could walk down a city street, find a shingle that says Psychic, pay the fee, and you'd have a stranger that never met you tell you if you've got something or not. Not that much diff from a shrink who doesn't know you.
I find it easier to look at what I need and find a path. Doing that is part of making one's personal owner's manual, which process in itself can help with what's needed.
If people don't trust my word on self-d, they prob don't trust me in other things.