Let's say we have a workflow variable in a workflow whose name is "due_date". In the first node of the workflow, a task asks the workflow initiator to fill-in the participants. He/She also has to set the due date, but a value is already suggested: The current date plus seven days.
To pre-set the due_date value, you can use an "input automation chain" on the node ("General" tab of the "Node Properties" dialog). In this automation chain, you use the "Workflow Context>Set Workflow Variable".
Fill in the following parameters: