Assignment for Start Event Node

No, by definition - a start form is not a task, and does not have an instance, and therefore there's nothing to "assign".  Whoever has access to the action or related action mapped to that process model will see the start form when they invoke the process, and the process instance is only initialized upon submission of the start form.  If they leave the start form prior to submitting it, it's gone.

No, you cant assign "User Input Task " on start Event as in Start event we assign task that is first executed.