In version 3.12.9, the ability to enter a Representative ID in the Export for CRA T4 became available. 

For earlier versions, this information needs to be manually added to the file before uploading it to CRA

Adding the Representative ID Manually

Below is an example of the text to add to the export file.

<TransmitterRepID>
    <RepID>ENTER REPRESENTATIVE ID HERE</RepID>
</TransmitterRepID>

Refer to the example below to see where to insert this text (rows 7-9).