@mhoye The public key is a deterministic function of the private one. That's how pubkey cryptography always inherently works.
The problem is that their js is asking for your private key and doing the derivation itself, rather than asking for your public key, because they want to paste your private key into the wg config file they auto-generate for you.
Rather than just letting you add a pubkey and write the config file yourself with the private key only you know.