Guitar amplifier into load box
First and foremost I would recommend consulting your owner's manual - amps with parallel output jacks usually have a list of common scenarios and how best to connect the amp.

Most parallel-out jacks on amps are switched, which means that if you only use one (and your manual should tell you if there's a specific one to use), the output transformer only sees the load on that jack.  It's not waiting for a second load on the other jack to meet that impedance.  So your simplest solution is to set the impedance to 16 ohms and run one line directly to your 16-ohm load box.

Just as an aside, stock PC audio cards are generally not very good for line-in recording.  You will likely encounter latency issues and lower quality vs a sound card or interface specifically for audio recording.  If you're just getting started, this won't necessarily be a problem, but just be aware of this going forward.