Hi Christina,
You can use the below example mentioned
SELECT-OPTIONS: so_date for sy-datum.
PARAMETERS: p_monat TYPE monat.
AT SELECTION-SCREEN on so_date.
IF not so_date is INITIAL.
p_monat = 3.
ENDIF.
the main thing i.e. after you press date you need to press enter, and rest will work as expected. I.e. after you press enter the p_monat field on selection screen will have value i.e. 3 and will be displayed on screen.
You can further manipulate as per your requirements.
Let me know in case any further inputs are required from my end.
BR/Thanks
Pranav Agrawal