According to the instructions here, you should fill out Payer Information by hand -- no explicit mentioning of UCI. Plus the payment receipt has the barcode on for verification, so there's nothing to worry about here... The only problem is in case they lose some (or all) of your application documents internally, they won't immediately determine to whom that belongs I guess...
I made the same mistake. Wasn't sure and left blank. But people told me on this forum that it is the same client id that is on the front your PR Card. But they will get it hoepfully from copy of your PR card which you provided. Submitted docs on June 30. Lets see...