By the way I found another way.

I tried the original way suggested and the result was OK. meaning can go next page. But I am supposed to be Blacklisted. So I tried signing up the UMobile Postpaid Plan, clicked check out and after adding IC number, the prompt came out saying

"Error: Sorry, you are unable to apply now as you have been blacklisted by service provider(s) below:
XXX
We welcome you to retry once your record with the above service provider(s) has been cleared."

End.