Linking Customisable Options?

I'm wanting to use customisable options for things such as disk space and bandwidth.

How do I link these options with the actual product, so for instance if someone enters "20" in the disk space box, WHMCS and cPanel auto-provision that much space?

This may be a really dim question, and apologies if it is, but I cant seem to be able to find where to link them!

Thanks