I had to pull together this information from a few Helm support docs and some confused searching around iPadOS settings. So, here is my guide to help you get Calendar setup.
Step 1: Open Settings and Search for “mail accounts”
Step 2: Select Other Account
Step 3: Select CardDAV or CalDAV
Step 4: Fill out server information
The important tidbit here is that the CardDAV and CalDAV server URL is https://helm.YOURDOMAIN.COM.