This will show you how to popup the save file dialog from selection screen. You can facilitate selection of file without manually entering file path.
Selection:

Result dialog:


DATA: gv_filename TYPE string,
gv_path TYPE string,
gv_result TYPE i.
SELECTION-SCREEN BEGIN OF BLOCK b4 WITH FRAME TITLE text-004.
PARAMETERS: p_path TYPE string LOWER CASE. "File path
SELECTION-SCREEN END OF BLOCK b4.
AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_path.
* Display save dialog window
CALL METHOD cl_gui_frontend_services=>file_save_dialog
EXPORTING
* window_title = ' '
default_extension = 'txt'
default_file_name = 'file_name'
initial_directory = 'C:\'
CHANGING
filename = gv_filename
path = gv_path
fullpath = p_path
user_action = gv_result.
Labels: cl_gui_frontend_services, file_save_dialog, open file dialog, Save file abap, Save file dialog