Honestly, just go use WealthSimple. I switched after this annoying unhelpful thread left me nowhere and its pay what you want and they have the exact drop down ...
Update: I just went over to Wealthsimple - which is Pay-what-you-want taxes and when you create a T2125 for each business, they AUTOMATICALLY pop up on your T4A...
This is a very stupid design. Why isn't there a way to assign my different T4As to different businesses? This is Self Employed Turbo Tax after all... shouldn't ...