Registers a wallet as a withdrawal destination. It starts as pending and becomes active once risk screening passes and ownership is proven by a satoshi test: send the exact test amount from this address to the account's own payouts wallet on the same network (GET /api/wallet?type=payouts) before it expires. A deposit to any other wallet does not count as the test. The test amount is issued shortly after registration — read it from GET /api/external-addresses/{externalAddressId}. Crypto only — fiat destinations are created automatically from the first on-ramp.