Bueno, leyendo la página del departamento del tesoro americano he leído que el formulario W-8BEN no puede ser tramitado por uno mismo, sino que se tiene que entregar a la Withholding agent, que imagino en este caso será ClickTrade. Dado que ellos no lo tramitan, imagino que es imposible que me retengan el 15% en vez del 30% en origen.
Aquí están las instrucciones del departamento del tesoro americano para rellenar el formulario, donde pone lo siguiente:
"Giving Form W-8BEN to the withholding agent. Do not send Form W-8BEN to the IRS. Instead, give it to the person who is requesting it from you. Generally, this will be the person from whom you receive the payment, who credits your account, or a partnership that allocates income to you. Give Form W-8BEN to the person requesting it before the payment is made to you, credited to your account or allocated. If you do not provide this form, the withholding agent may have to withhold at the 30% rate"
http://www.irs.gov/pub/irs-pdf/iw8ben.pdf