Within Teamwork.com, you can change the creator of an individual task.
Go to the List area of your project and select the relevant task to open the task details view.
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When viewing the individual task, click the options button (three dots) in the top right and select Modify Properties from the dropdown menu.
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In the modify modal, you can select a new creator from the dropdown. You can pick any user who has permissions to create tasks on the project.
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Click Apply Changes to update the creator of the task.
For more information, see: Changing Completed Task Details