When possible, it is better (and quicker) to download the media rather than record it. There are several free plug-ins available for Firefox and other web browsers for downloading YouTube videos - use you favourite search engine to find one for your web browser. Note that copying, recording or downloading some on-line content may be prohibited by copyright.
Do you have a tower computer with separate Line In (blue) and microphone inputs? If you do have a separate Line-In, you can set the Audacity recording device to Line-In.
If you do not not have a separate Line-In, it may be a bad idea to to connect audio hardware emitting strong signals like a synth to the mic port. You could get mono sound or distortion.
If you want to do it, you will probably need to enable “Listen to this device” in Windows Sound which will play the device through the speakers. Then record the speakers with WASAPI loopback. That “might” give you better results than recording from the mic input.