This should be possible if you use the UNC Path for OneDrive.
If you open your OneDrive on the Web you can see an URL like this:
You have to convert the URL to an UNC-Path and shorten it to the library URL:
This should work. If not just add a @ssl at the end of the Domain Name:
After you figure out the UNC Path, test connecting to it in Windows Explorer by entering your OneDrive credentials when asked.
If this works, FileUltimate would be also able to connect to it with this code:
var rootFolder = new FileManagerRootFolder
Name = "My OneDrive",
Location = new PhysicalLocation
Path = @"\\tenant-my.sharepoint.com@ssl\personal\username\Documents",
UserName = "USERNAME",
Password = "PASSWORD"
Let me know.