Oh I see, you mean automatically select a payload...
The problem is the order that the files are listed in is 'random' - it depends on how they are enumerated by grub2.
Adding or deleting files would change the order.
agFM is NOT a menu system, it is a filesystem browser.
You would have to specify the exact partition, path and filename + timeout.
I will suggest it to a1ive but he may just reject it as not appropriate.
You can place a .cfg file at the root of one (or both) partitions.
2. Select one of the USB partitions
3. Select the Windows 10.cfg file
You can have multiple .cfg files as 'shortcuts' to other files or folders.
Once agFM has become more stable, I may develop a menu system for it (rather than a File Manager system) but that is a while away yet.