Proper syntax for "Custom Options" in GUI

[hikeonpast]

Using the GUI, I'm attempting to do a batch convert. I'm struggling to find the correct syntax to use in the Profile->Video Codec->Custom Options field.

I know which command line directives I want to pass in (rate=30.0 in this case), but I can't locate the correct formatting to use command line options in the GUI. I found this information once and it worked flawlessly, but I can't find my notes and the online documentation of this specific issue seems very poor.

Thanks!