There is a tool available for formatting the code snippet in OneNote — Note Highlight project —

Well and good; with some added complexity of installing the plugin and so on…

Is there any alternative?

Well, not the code syntax formatting the way the plugin does but a properly indented code with a decent readability — see if that suffices for you.

Step 1

Paste the code into a beautify engine such as,

Beautify the code on the site. Copy this code.

Step 2

Paste the code with Keep Source formatting option: