First, click on Extra Vendors
Then Add your extra vendor
You should have this:
Now that you have added your extra vendors, you can add them to your source.
Extra vendors in your notice
The extra vendors will be displayed in a list, and the consent button will be displayed for each of them.
For a web notice: (Classic template)
For a mobile App (iOS or Android): (Classic template)
The extra vendors will be also displayed below the purpose you assign them.