Your best bet for now is to run the task as a command line with something like >C:\DuplicatiOutput.txt at the end of the line.
So if you are wanting the output of a backup job piped to a file, then I’d suggest using the “Export → As Command-line” feature to get a command line you can then add the above redirect to and run in a shell.
If it’s something else, such as a Compare, that you want then you’ll have to manually build the command line. It’s not difficult once you understand it, but getting there can be a bit of a pain.
Let us know what the specific command you’re looking to log is and maybe we can put a sample together for you.
It’s POSSIBLE (but I’ve not tested) that the Windows 10 bash shell will work with STDIN @Kahomono described.
I’m not going to disagree, but I will point out that the way to do what’s being asked for is to use the command line…which I’m pretty sure is what people used to rag on Linux about… 
Also, in case you didn’t catch it I’ve started a “Features” topic about getting a --stdout=<path> type advanced parameter added. 