Tell your client that as you are paid via Upwork this is already on file with Upwork. Personally I would not fill in anything for any prospective client... Do you have a contract with that client?
As Petra, stated we recommend to not give any personal information to clients. Your client should not need your W-8BEN Form. You have already provided this information to Upwork. If the client has any questions on how payments and taxes work they can submit a ticket with customer support or review the help article here.
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