With Ford Transit vans starting in MY 2020- if you did not order the OEM Factory controller there is NO WAY to add one. Ed Bowers, aka crewvanman is a super knowledgeable sales person at Gary Crossley Ford in Kansas City. He verified this after speaking with his contacts at Ford.
The best solution is the Curt Echo, and that is what I am using on my Transit Van. I do have the OEM on my Expedition and knew they were fairly easy to add it it didn't come with one, yet that seems to have all changed starting with 2020 models.
Met someone recently towing with a 2022 Explorer ST and just like my van- he had the HD Tow Package, yet NO OEM TBC.
Below is a link with 221 posts on the Ford Transit Forum about this topic. If you start with the very last post, keep in mind the congratulations for getting it to work is for a 2019 van.
Wow! So glad you got the TBC working. Do you have the Part Number for the Plug/Adaptor/Pigtail that fixed the connection issue? Thank you
www.fordtransitusaforum.com