ahh I just messed around with so called dapp website and understand what I think you are asking.  Anyways I'm not going to spend a lot of my time with it because more than likely you will get a better rate buying on one of the exchanges.  It would appear metamask needs the contract address for the CRB token and of course the some amount of ETH needs to be present.  Hopefully a team member will explain how it works better.