Inline and Defer CSS? setting Autoptimise

I’d appreciate some advice regarding the Inline and Defer CSS setting in the Autoptimise plugin. Should I:

a) Check the box to inline and defer and add include the ‘above the fold css’ in the box below? or

b) Check the box to inline and defer, but leave the include the ‘above the fold css’ in the box below blank and leave Autoptimize criticalcss.com power-up to do the rest?

TIA