Creates a DECLARED self-hosted counterparty for userId + application. For subType OWNED, optional signature and signedMessage verify wallet control; the signed message format and UTC date rules are enforced server-side (embedded DD/MM/YYYY must be yesterday, today, or tomorrow in UTC).

Example signedMessage (replace address, userId, and the date so the date is valid in UTC):

I am verifying ownership of the wallet address {address} as customer {userId}. This message was signed on {DD/MM/YYYY} to confirm my control over this wallet.

Sign it with the wallet that owns address, then send the resulting signature with the same signedMessage text you signed (see request body examples).