Trigger a popup before of parameters of selection

Hello,

Actually I am creating a program where I have certains parameters of selection. I want to execute first a popup when the user open my ZTRANSACTION but I cant to get this action, the popup diplays after that the user fill fields and press execute button. I puts my code to popup at START-OF-SELECTION but it not works, Any sugerence? I use the FM POPUP_GET_VALUES for my emergent window.
Thanks!