Check this topic
https://discourse.osmc.tv/t/solved-i-cant-get-tranmission-to-work/1309/10
everything gets automounted in /Media
You want I assume a fixed mount
This explains also how How To Mount A USB Flash Disk On The Raspberry Pi - Raspberry Pi Spy
(it only skips one thing in our OSMC case, you need to unmount it from media first)