When you use initiate-trade, you give us an output address (in bitshares, this will be an account name), and Blocktrades gives you an input address that is linked to that output address. Just send some currency to the input address that initiate-trade gave you, Blocktrades will convert it and send it to the output address you originally gave us.
That input address will always be linked to the output address you gave us. If you want to send to a different address/account, call initiate-trade again with a different outputAddress.