We proceed through the list of xpub-generated addresses and withdraw once to each until all 500 are used once. You cannot choose or skip addresses for withdrawals.
It’s unlikely we'll ever allow this for users because of the gap limit on most wallets. Most wallets stop scanning after 20 empty addresses. Giving our customers the ability to cycle through addresses could potentially create address gaps such that received Bitcoin will appear "lost" on most wallet interfaces.
If you have accidentally already used an address generated by the extended public key, then it will get used by Swan automatically. If you wish to avoid this address reuse, then we encourage you to set up a Swan-specific wallet, or at least a Swan-specific sub-account within your wallet with its own child extended public key. Then only use that sub-account for receiving withdrawals from Swan.