When you have income reported to you on a T4A slip, you need to enter your T4A first, then enter your expenses on the T2125 - Statement of Business Activities which has been created from your T4A entry.

Enter your T4A slip by going to Income > T-Slips > T4A - Pension, Retirement, Annuity. After you enter either Box 20 or 48 from that slip, you will need to go to Income > Self-Employment > Self-Employment Profile. Here you should see a check beside the box Professional, Commission and/or Other type of business. 

Continue to the T2125 Business Summary page where you can choose to "edit" the business which shows your T4A income.

I have a similar situation income for online travel discounts but the T4A the commission is in box 028 and I have expenses?  What is the process then?

@Jasmin- you will do the same as Kim said above and you will be able to continue in the T2125 and enter your business expenses.