f4 help in the selection screen from a int table

I collected the datas from the function module. and the internal table is populated.

the internal table contains manny fields.

among these fields i need to take only one field and this field value to be used for F4 help in the selection screen ( not a dynpro)

for example : in the selection screen

I have to select the PO.

based on the user i have filtered the PO. (determined dynamically by using the user name this i have done it)

then i have to assign the PO the selection screen.

which FM will be better to use for this type of scenario and help me out with some example codes particularly for my req

Thanks