project name field in PR page

We created the Project name and Task name field as DFF in "Manage Project Costing Descriptive Flexfields" to bring project name and task name fields to Purchase Requisition page.

 Client is expecting that when they select project number, the project name need to be selected automatically. Similarly task name to be selected based on task number.

 But when they select the project number and task number, the project name and task names are not selected automatically.

 Please help us to resolve this issue.