If want to use a pop-up, the simplest is to use Show Custom Dialog and specify two global date fields as input field 1 and input field 2.
Then this script will find your records:
Enter Find Mode 
Set Field [yourtable::yourdatefield; globaldatefield1 & "..." & globaldatefield2]
Set Error Capture [on]
Perform find 
Set Error Capture [off]
The one problem with using Show Custom Dialog is that no validation is automatically performed on the input fields. It may be better to place the two global fields next to your button and dispense with the pop-up.