Do I need to sign w8-ben form on IBKR?

Yes you're right, you do need to declare.

"You filled in the W-8 Ben form online when you opened the account with us. You can check it online if you proceed to Client Portal -- Settings -- Account settings -- Profile and click on "i" icon and scroll down to tax forms."

Do make sure the info there is correct and tax ID should be your NRIC.
There is a W8-BEN downloadable tax form on the internet but you don't need to upload that, just fill in the declaration on IBKR.

Hope this helps! :)