You can only use inputs the are supported by the operation, commonly documented in the operations Description tab. If the capability you need is not there you can always use other methods such as using other operations supporting the target service or by creating a new operation.

For AD user with description or any other field in AD user form you can fallback to PowerShell Script operation in the Base content pack. Use New-ADUser command to create the user with descriptions and other options. Make your user name and other options as a flow variable and there you have your reusable subflow for the future.

New-ADUser -name "${newUser}" -description "${description}"

Thanks for the quick reply, I'll test this and give feedback