There once was a way to generate CMYK pdfs via an extension, but that doesn't work anymore. The built-in functionality is for previewing colors on screen, and for usage and import into Scribus, but as far as I could find, it doesn't work very well, and is generally really hard to use.
Currently, your best bet is to draw in RGB, or to set a color profile and choose your colors from that (sometimes doesn't work well), but then to move your drawing into another program for conversion to CMYK.
There is currently an effort going on for planning how to do CMYK with Inkscape, and to fund this development with user donations. It's going to be focused on export functionality. It will require a lot of user engagement when it is made public, and it will require new volunteer developers to do it. If you know someone who would be interested to help, please ask them to watch out for news about this on the Inkscape website, or, if they are a developer, to contact Inkscape developer via their mailing list.