If you open an xkcd link, e.g. https://xkcd.com/899/, you will see a note at the bottom with the link. For this one, number_line is in the link for the image. Use the \xkcd{} command, note that you need to use \detokenize{_} so underscores are interpreted appropriately. E.g. \xkcd{number\detokenize{_}line}:

Also note that this does not work in the PDF.