Like MortenMacFly said, there is no need to "force" anything.
Different settings are just used differently:
"Other option" : whatever you write in this box is passed on to the compiler as a command line option, as you type it.
"#defines" : you write symbols (e.g., ENABLE_THIS or SOME_FLAG=1) and these symbols are translated to the -D syntax (-DENABLE_THIS and -DSOME_FLAG=1 per examples), and then passed on to the compiler in the command line.
Use whatever approach you're more comfortable with, but it's advisable to separate #defines from compiler arguments for clarity.