Glad to see you here in the Community about integrating a third-party app. I can provide information about syncing apps with QuickBooks Online (QBO).
Third-party applications integrated with our program are available in the App Store within QuickBooks. As of now, the SiteMinder app isn't listed yet (see screenshot below).
SiteMinder apps have a list of integrated applications on its website, specifically in the Marketplace section. I recommend contacting their dedicated support to check how to connect or share the data to QuickBooks.
There are also several third-party applications that can be integrated with QuickBooks Online to help you in automating your workflows. You can go to your Apps menu or use this link to see the details of the apps: QuickBooks App Store.