You can use a domain that you purchased on Crazy Domains to point to your OfferingTree site by setting what is known as a CNAME record. Here are instructions from Crazy Domains to set up a CNAME record:
For the subdomain, please enter www and customdomains.offeringtree.com for the target name.
If you already have a CNAME record for www, edit that one instead of adding a new one.
Once you have set this up, please contact us at firstname.lastname@example.org and let us know the domain you would like to use, and we'll finish the setup on our end.