Pass Importing Parameter of Method from Workflow --> Task

Hello Experts,

I am beginner to SAP workflow i am having problem regarding passing Importing parameter of method from workflow to task.

I am using activity with Task defined and in that task Business Object USER and method GETDETAIL is used to get user details.

Here method GETDETAIL can not determine importing parameter USERNAME so user details are not fetched, but if i pass initial value of Workflow Container element USER Object Type USER then method can fetch user details.

Please tell me how i can pass that methods importing Parameter.

Thank you in advance.