Sorry for the confusion, but we are not sure we understand the desired workflow. If you need a scheduled task, user will have to be predefined in the Activity Scope. It is not possible to specify the users before running a task. However, if you want to use a custom command, you will be able to select the users manually (can be one or multiple users). If using a custom command is what you need, please, specify whether the computers OU must be predefined or there should be an option for selecting it.