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 email@example.com and let us know the domain you would like to use, and we'll finish the setup on our end.
Redirecting the naked domain
In addition to using www.<your domain> to point to your OfferingTree site, it is also advisable to redirect the main domain to the www version of your domain. This will allow users to type in your domain on a browser and automatically be redirected to www.<your domain>. To do this, you can use Siteground's redirect tool https://tools.siteground.com/redirect