Every order page carries the same details as text. Most Android phones give you one field and want the whole activation key: copy it from the order page and paste it in.
Some phones ask for two separate lines instead. Those are on the page too, as the SM-DP+ address and the activation code.
On an iPhone: Settings → Mobile Service → Add eSIM → Use QR Code → Enter Details Manually, then paste the two values.
Copy and paste rather than retyping. The key is long and one wrong character fails without saying why.