How do I link a T4A to my existing small business?

Income reported in box 20 of a T4A will report as commission income and claimed on the self-employment schedule for a commission based enterprise. You will need to create the Commission Business Schedule and claim expense against this income.

Thanks for your reply Terry.  I have been reporting this commission on the Business Schedule for several years.  What is new is that the company started issuing T4A in 2016.  When I enter this amount in the income field as a T4A, then it pulls thru to the Business Summary and appears like another business, but I already have my business there because I did a Carry Forward of my return from previous year. So it looks like 2 businesses.  I'd like to know how to link the two.

Unfortunately you can not link these separate schedules. You could close the existing business effective the beginning of 2017 and enter all of the expenses on the new commission related business schedule. You should clear out any UCC amounts simply by entering a disposition at the UCC and then transfer the UCC amount to the new commission business.
Some one else might also suggest just deleting the previous T2125 and re-enter all of this information in the new schedule as if the business was already existing. An acceptable explanation to CRA is that you were forced to change business type mid-stream due to the reporting obligations of this new income source. More of a paper shuffle then any thing but I understand this is an inconvenience to you.

Darn.... I'm not very proficient so am reluctant to try to copy everything over though this may be my only option.  What is a UCC?

Undepreciated Capital Cost of assets such as equipment and automobile. Found in the Auto expenses and Capital Cost Allowance sections. You could also pay for Turbo Tax first, download the PDF copy of the tax return, and have a print out of the exiting T2125 business schedule that may help to create the new Commission Self-Employment schedule. Then delete the old before actually Netfiling the tax return.

In order to access a full PDF copy of all forms and schedules you will need to pay for the tax return first under the "File" tab. Once you have completed the required information in your "shopping cart" you will have access to the PDF version. You do not have to Netfile the tax return until after you are satisfied it is complete.
Access  the PDF under the "Review" tab > Bottom Line (tax summaries) and below the "Magnifying glass and paper" will be the "Review T1 General in Blue".

So I have to transfer all of my tax information from my 2125 into the new company created by the software?  And then delete the old form?

I had the same problem, but I just had to move my T4A income from the Box 48 labelled "Professional Fees" to the Box 48 labelled "Business Fees". Then the T4A income linked to my existing business.