The code uses the official click-to-chat link format (wa.me followed by your number in international format). The phone opens WhatsApp, or the web version on a computer, with your contact selected and the message you chose already in the box.
Write the number with the country code and without the leading zero, plus sign or spaces: an Irish mobile 087 123 4567 becomes 353871234567. A pre-filled message like "Hi, I saw the code on your van" tells you where the customer came from without any tracking.
This is a link code, so it stays small even with a message. It is a good candidate for the speech-bubble frame with your call to action, and for a green that is dark enough to scan: the WhatsApp brand green on white passes the scan check, lighter greens do not.
Questions
Does the person need my number saved?
No. The link opens the chat directly. They see your WhatsApp name and can message immediately.
Can I use a WhatsApp Business number?
Yes, the format is identical. Any number registered on WhatsApp works.
Can I see how many people scanned it?
Not with a free static code: there is nothing in the middle to count. Lifetime editable codes include a simple scan counter.