There are several ways:
Show Custom Dialog is a script step that can pop up a dialog box with up to three input fields for entering data such as the week number.
Fields on a layout can request this info--most likely fields with global storage specified. You can change to this layout and then clicking a button, pressing enter or another user action takes the user back after they have entered the data.
OR you can use new window to create your own custom dialog using a layout in this window to request this info. This takes more scripting, but unlike a custom dialog, it allows you to use a value list and other formatting not possible with show custom dialog.