Getting the public key set for your zero knowledge proof of ownership.

Making proofs (as described on this page) requires you to have the right set of public keys, derived from taproot utxos that existed at a specific block height.
On this page there will always be a link to the latest utxo keyset file in use by this server. Proofs should be generated with that keyset. They were generated from the utxo snapshot at the specified blockheight. As long as you had a taproot utxo at that specific blockheight for which you own the private key, you can generate the proof.

Download mainnet keyset here
Download signet keyset here

Only mainnet is currently working. Note that the minimum amount of sats (500000) and the blockheight of the snapshot are in the file name.
Trusting the server to generate the keyset file for you is not great! It's just to make the proof of concept easier. While it doesn't violate the security of your private keys, it theoretically makes the anonymity of your key weaker, if the server is malicious.
You can generate the keyset yourself, with your own bitcoin node. Read here for how to do that; note that it will take 30-40 minutes at minimum, but for the technically capable, it's not too hard, if you have the motivation.