You can define a selection screen parameter with the below syntax
PARAMETERS: p_var1 TYPE i DEFAULT 100,
This parameter will auto defaulted with value = 100.
Parameters cannot have data type F. The data type F is not supported in the Selection Screen
To hide an existing parameter field, use NO-DISPLAY option
PARAMTER P_VAR2 NO-DISPLAY.
To make a parameter as mandatory input field, the OBLIGATORY option is used.
PARAMETERS: p_var3 TYPE i OBLIGATORY.