we have to check with PSYST table and if IINIT indicator is 
yes then we can default the value in PBO module.

WE CAN DEFINE IN SCREEN FIELD ATTRIBUTES BY GIVING ID IN 
SET & GET PARAMETER

Default values can be handled using the event INITIALIZATION.

Parameters: field_name = 'default value'
select-options: the field declared using select options is 
similar to an internal table with 4 fields - sign, option, low 
and high. 

In the Initialization event, we can set field_name-low = 
'default value1' and field_name-high = 'default value2' so as 
to display the default values in the selection screen.