Define two fields with global storage specified. You can define them in any table in your database. I'll call them Globals::gDate1 and Globals::gDate2
Show Custom Dialog ["Please enter two dates" ] //specify Globals::gDate1 and Globals::gDate2 as the input fields for the dialog.
If [ Get ( LastMessageChoice ) = 1 // Ok was clicked ]
Go to Layout ["ShippingReport" ( ShippingData ) ]
Enter find mode  //clear the pause check box
Set Field [ShippingData::ShipDate ; Globals::gDate1 & "..." & Globals::gDate2 ]
Set Error Capture [on] // keep "no records found" dialog from interrupting the script
Perform Find 
Sort Records [No dialog ; Restore ] //sort records if necessary for your report
You may also want to consider designing a layout with the two globals fields instead of using show custom dialog. You can then enter the two dates and click a button to start the above script. This allows you to enforce valid date formats at the time the user enters dates. If you use the New Window script step, it is possible to open a "dialog box" with your own layout design.