set default value for list box in tandard screen

hi sap experts.

i add an input /output field to my screen and set dropdown as 'listbox' .

i create a z table and insert some records to it.

in my main table (mara) i create an append structure and add a field to table and in foreign keys i refer my field to z table ,so in my screen when i select my list box i see the values in z table.

but now i have a problem,how can i set my list box whit a special value in z table?

thanks for your helps.