To make Custom Smart Lines note-attach like the preset ones in Finale, Option-Click (Mac) or Alt-Click (Win) the Tab Slide Tool:
…and choose your Custom Smart Line:
*Any* Custom Smart Line can be note-attached by assigning it to the Tab Slide. You can “assign to apply” as you need to:
The first example is a repeated font character; the second is a wide dash with one of the preset Arrow heads.