CertCapture has a dedicated menu in the Avatax add-on menu. You have to setup the connection credentials to CertCapture, completing the information indicated in the setup window, as follows: The actual API URI for CertCapture is https://api.certcapture.com/v2 CertCapture doesn't have a sandbox site, you can use the same CertCapture account for testing and production purposes (as you are not entering transactions to CertCapture, but to the Avatax portal, where you do have the option to use a sandbox account). After you set your credentials, and save the data, you can test the connection to validate it works.  

Configure your API credentials

Your CertCapture credentials requires API access permissions. To configure this login to the CertCapture site, go to your profile, and define the credentials for the REST API Access: Your CertCapture Company ID is visible from the Company Details section. This is the Id you must complete in the Setup Company Id field.