One thing I wonder about is what the term “API” refers to. PowerShell (or a batch file) can also go about Using Duplicati from the Command Line which is documented. A question is – what schedules backups?
If the automation scripts do it and are able to notify you somehow when backup starts, then you have all that’s requested except for a list of files transferred – but these files are not source files. Is info relevant?
If the goal is to have Duplicati’s scheduler schedule jobs, then it’s harder because server is independent of command line. Here are some ideas for ways to manage backup jobs done by server, using scripting.