The easiest way I find is just changing the BIOS/uEFI/firmware default so it boots with numlock on by default. I’ve not yet encountered a box where this doesn’t work, but its possible not all firmware has a setting that allows for this.
I believe I have a box that defaults to turning on without numlock configured, when I next use it (and re-discover which box it is; its something I only rarely use) I’ll have a look and possibly return with more details (having something to configure myself I guess).
FYI: The manual link you provided was for the stable release; ie. Lubuntu 22.10, but you mentioned 22.04 or the lts release. To read/use the correct manual; you just need to change the word “stable” in the URL to be “lts”. You’ll find the manual links documented here as they can change over time.