I can add variable with User or Group type and mark it as Show on WF Start form.
But how can I allow for users choose only 1 user?
I don't see that you have much control over the People control on a auto-generated start form based on a workflow variable.
But you could customize the start form with a Nintex Form. Then from there you can choose if the People control can allow multiple or not.
I hope somebody know any Tricks... bcs we have no Forms
Retrieving data ...