I’m guessing you’re talking about the newest (2018) iPad Pros. If you have a newer iMac with USB-C or Thunderbolt ports, you can use the USB-C charging cable that comes with the iPad. If not, you’ll need a USB-C to USB-A adaptor.
As already stated, the assumption is that you have the 'new' 2018 iPad Pro w/ the USB-C charging port - correct? If so, checkout Apple's KB Article - also, please tell us what your purpose(s) may be - my guess is that you want to use iTunes on your iMac - if you have other thoughts, give us some idea(s) - the USB-C port on the iPad Pro does not have the many features of standard USB connections, e.g. connecting an external HD is not an option. Dave