If it has pre-out use that. The first circuit inside the sub is a filter, so you do not need a sub out, the full range signal will be fine.
If it doesn't then you need to use a LOC- Line Out Converter. They are cheap and work great https://www.parts-express.com/stinger-sgn111-30-watt-fixed-line-out-converter--268-482
This is one time where it does not pay to spend more. All you get for more money is better looks. The circuit inside is nothing more than a pair of resistors. These drop your speaker level voltage to line level allowing your amp to drive the sub from the speaker outs. If you are interested in tech here is exactly what it is and how it works. https://www.epanorama.net/circuits/speaker_to_line.html
If you are handy you can use that article to DIY. That's what I did, and put the circuit inside my amp as a mod. https://systems.audiogon.com/systems/8367 Grand total parts cost me less than $2 for the 4 resistors. But if you want fast and easy you can simply buy the LOC and be done with it.