Function block in S7-300

I work on siemens PLC(S7-300).I work with Function block.I don't
know how can I force one of inputs to high(TRUE) or low(FLASE) (for example EN input in function block)?please let me know how can I do that?

I think I understand your question. Make yourself an 'always on' flag by uncoditionally setting it.
Alternatively "TRUE" and "FALSE" are acceptable keywords that Step7 will accept.