Add User to To-Do-List using server side C#

You need to set the To Do list by adding Roles to the To Do list property of the Stage of the Folder. It is not something that you do 'manually' in code, but a function of the Roles property.

We have a great many examples at the link below that demonstrate the use of both static (pre-set) and dynamic (changing at runtime based on current data) Roles. I suspect from your question that you are needing a dynamic role. If you could describe the situation in more detail, I am sure we could provide a simple example.