You can use a domain that you purchased from SiteGround to point to your OfferingTree site by setting what is known as a CNAME record. Here are instructions from SiteGround on how to set up a CNAME record: https://www.siteground.com/kb/site-tools-vs-cpanel-comparison-create-cname-records/
For the name field, please enter www and customdomains.offeringtree.com for the CNAME field.
Ensure that you don't already have a CNAME record for www and there are no conflicts. If you already have a CNAME for www please edit that one instead of creating on. 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.