In order to provide a proper answer there need to be some specifics clarified.
Do you need just the code that sends F3 for instance or you need it as a prefix for a certain barcode?
Anyway you can use appendix from any our DS series handheld scanner manual. There is a list of ascii values for all keys.
F3 would be 5003.
a prefix/suffix is not enough, it should a secquence of keys (like mentioned) or alternatively a singe key (F3, F1, TAB etc...) as barcode.
the sequence would be favoured.