1. You have to mount your USB stick first with BH Device Manager
(If not already and I think you are you have picons on USB)
you can use also FAT32 windows format on your USB stick for Timeshift and Picons
(FAT32 format It's better because you wont have a problem with the image boot)
2.System Menu>>Recording paths Panel>>move (go down) to Timeshift location >>press OK>>In Select Location PANEL>> press Menu button>>switch to filelist and move to USB Stick folder
>>Blue button (Add Bookmark)>>OK
Now you can make choose in Recording paths panel Timeshift location.