That's if you want to go with Noctua for getting the kit. The requirement is AM4 CPU/motherboard invoice and your Noctua invoice or photo with your address on the Noctua cooler.
Alternatively, OC sell it themselves (this is for the D14 and other coolers, so please ensure that you choose the right kit as there are multiple kits): https://www.overclockers.co.uk/noctua-nm-am4-mounting-kit-hs-037-nc.html
Thanks for the link, I have a NH-D15 on my old i5-2500k system which I can use and get a standard fan for that as it will be relegated to clerical work and won't be overclocked
Its worth paying the £6 for the kit rather than have to wait until after I have bought the new system and have to wait for it to arrive