Using line-in on a combo headphone/microphone jack

Hey folks.

Sorry if I'm posting in the wrong section but I'm finding the Dell website a bit hard to navigate and wrap my head around.

Anyway, my question:

I have an Inspiron 13z, the N311z version. It has a combo audio jack, that is, headphone and microphone. The problem is, I don't know how to switch it to line-in. I have a suspicion that it is automatic and the computer tries to detect whether the line is a microphone or headphones, but I'm looking to run something line-in: that is, it isn't a microphone per se. I've looked around the audio options and the Dell support centre isn't particularly helping me find any answers.

Any input would be appreciated.