In the T4A form in the software, go down to "Other information" - click on the triangle to have access to the drop down menu. From the list, choose the corresponding box. Just below "Footnotes relating to specific T-slip entries and Box 135 (RL-2, Box 235)" - click on the triangle to access the drop down menu.
If you purchased UFile 4 box at a retailer, you should have a code inside the box for UFile Online. You will have to enter that code when you will be asked to pay (usually step 4).