Brokerage company requested more info about my W8-BEN and made withhold backup under W9

Hi! I'm a non-US resident, only visit the country as tourist some weeks per quarter. I hold a US bank account and a phone number. Opened a brokerage account to buy stocks/fixed income but the brokerage company invalidated my W8-BEN and requested additional forms showing that I'm not a US resident and qualify to W8-BEN. Meanwhile they made a backup withhold of 24% in all my operation under W9 form. Now they approved my W8-BEN and stoped backup withholding on capital gains. My question is: how do I get a refund from IRS of the amount they withheld under W9? I asked them to refund it but they said that's not possible because the amount is already sent to IRS.

Thanks for your attention!