hello @HB8 ,
I do believe that if you want to Add a To-Do, it's not possible to assign it to other users, cause it's a "personal" list.

If you want to collaborate with other users, please use the Planner.
Please let me know if the issue is fixed.
If I have answered your question, please mark my post as Solved.
If you like my response, please give it a Thumbs Up.
I may not have all the answers, but I'm curious and passionate enough to keep looking for answers and to keep sharing them.
My blog site about Power Automate: https://powerautomatejoy.com/
BR,
Marco