Variable popup screen within BEx workbook

Hi

I have a function which requires the user to enter the start and end period. Based on this selection the system shall perform certain calculations only for these selected periods. In IP i am handling this start and end month with a variable. However when i use a workbook in the BEx analyzer and execute this function within the workbook, the system gives an error that the variable values are not entered. How can i call the variable input screen to pop up in the BEx analyzer workbook.

A quick help shall be much appreciated

Cheers

Arvind