Mar 6, 2018 11:17:43 AM by Will G
Mar 6, 2018 11:33:04 AM Edited Mar 6, 2018 11:34:39 AM by Petra R
Ask your client to ask Upwork directly.
I assume he means an (F)EIN which is the (Federal) Employer Identification Number which he can not use because neither you nor Upwork act as an employer or employee.
Upwork Support can, if appropriate, give him their W9