In case none of the recording-sources reflects equivalence to the “Stereo Mix” recording-source, simply connect your “Line-out” port to “Line-In” port through a suitable audio cable, and select recording-source in the software as “Line-in” or equivalent to it, and hope this should accomplish your requirement.

If you use “Line Out” port, then you need a simple audio splitter like this:

Stereo cable (3.5mm 2 male pin audio stereo plug)